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OUTLINE  OF  GENETICS 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


THE  BAKER  AND  TAYLOR  COMPANY 

NEW  YORK 

THE  CAMBRIDGE  UNIVERSITY  PRESS 

LONDON 

THE  MARUZEN-KABUSHIKI-KAISHA 

TOKYO,  OSAKA,    KYOTO,    FUKCOKA,    SENDAI 

THE  MISSION  BOOK  COMPANY 

SBANSHAI 


OUTLINE  OF  GENETICS 


WITH  SPECIAL  REFERENCE  10 
PLANT  MATERIAL 


BY 


MERLE    C.  COULTER 

Assistant  Professor  in  Plant  Genetics 
in  the  University  of  Chicago 


THE  UNIVERSITY  OF  CHICAGO  PRESS 
CHICAGO,  ILLINOIS 


Copyright  1Q23  By 
The  University  of  Chicago 


All  Rights  Reserved 


Published  April  1Q23 


Composed  and  Printed  By 

The  University  of  Chicago  Press 

Chicago.  Illinois,  U.S.A. 


PREFACE 

Probably  no  phase  of  science  has  ever  de\'eloped 
more  rapidly  than  has  the  subject  of  genetics  during  the 
last  decade.  The  number  of  competent  investigators 
has  so  increased,  the  scope  of  investigation  has  so  broad- 
ened, and  the  methods  have  so  improved  that  there  exists 
now  an  extensive  literature  on  the  subject.  The  rapi(Hty 
of  its  publication  brings  repeated  changes  in  intcrj:)reLa- 
tion  of  the  phenomena  of  heredity,  and  keeps  the  subject 
in  a  state  of  flux.  For  this  reason  it  is  difficult  and  per- 
haps even  dangerous  to  prepare  a  textbook  on  genetics. 
Some  of  the  views  expressed  in  the  manuscript  may  be 
out  of  date  when  the  book  is  issued.  It  is  evident,  there- 
fore, that  the  material  of  the  present  text  can  represent 
only  one  author's  interpretation  of  the  status  of  genetics 
in  1923. 

Plant  Genetics,  by  John  M.  Coulter  and  Merle  C. 
Coulter,  was  published  in  191 8.  The  present  text  is 
more  than  a  new  edition  of  the  earher  one,  since  it  repre- 
sents a  thorough  revision  of  the  material  presented. 
The  former  title  was  felt  to  be  an  unfortunate  one,  since 
it  seemed  to  imply  that  the  genetics  of  plants  is  some- 
thing different  from  the  genetics  of  animals.  Since  the 
fundamental  principles  of  inheritance  are  the  same  in  the 
two  groups  of  organisms,  and  since  it  is  necessar\'  to  use 
many  of  the  results  of  animal  investigation  to  illustrate 
certain  points,  it  is  felt  that  a  more  appropriate  title 
for  the  present  text  is  Outline  of  Genetics,  with  Special 
Reference  to  Plant  Material. 


1 71 79 


vi  Preface 

The  primary  object  of  the  text  is  to  meet  a  definite 
need  felt  by  botanical  students.  Such  students,  in  their 
contact  with  current  botanical  literature,  frequently 
encounter  papers  dealing  with  the  genetics  of  plants, 
and  through  lack  of  preparation  are  unable  to  grasp  their 
significance.  Since  this  literature  is  far  too  important 
to  be  neglected,  it  was  thought  advisable  to  provide  such 
preparation  in  the  simplest  possible  manner.  In  addi- 
tion to  this  need,  it  is  felt  that  the  text  will  be  useful 
to  biological  students  for  two  important  reasons.  In  the 
first  place,  the  presentation  is  simple  enough  for  students 
with  little  or  no  biological  background  to  understand; 
and  in  the  second  place,  the  subject  is  brought  more 
nearly  "up  to  date"  than  in  any  earlier  text.  This  is 
especially  important  in  view  of  the  numerous  significant 
investigations  that  have  been  made  during  the  last  year 
or  two. 

In  order  to  adapt  the  text  to  a  greater  variety  of 
needs  and  interests,  an  arrangement  of  material  is  made 
by  means  of  which  it  may  be  used  either  as  an  elementary 
text  or  as  one  somewhat  more  advanced.  The  material 
presented  in  large  type  provides  a  simple  account  which 
may  be  read  coherently  without  reference  to  the  material 
in  smaller  type.  The  latter  material  presents  a  more 
intensive  treatment  of  certain  phases  of  the  subject,  and 
will  be  of  interest  and  value  to  those  who  wish  to  work 
out  more  thoroughly  some  of  the  details  of  investigation 
or  application. 

M.  C.  C. 


TABLE  OF  CONTENTS 

CHAPTER  PAGE 

I.  The  Background  of  Genetics i 

II.  The  Inheritance  of  Acquired  Char.\cters     .       .  12 

III.  Mendel's  Law 37 

IV.  The  Factor  Hypothesis 56 

\'.  Inheritance  of  Quantitative  Characters  72 

VI.  Linkage q6 

\TL  Mutation 109 

VIII.  Bud  Variation 119 

IX.  The  Gametophyte  in  Inheritance  .       .128 

X.  Sterility 131 

XL  The  Endosperm  in  Inheritance         .       .       .       .  14^ 

XII.  Hybrid  Vigor 156 

XIII.  Sex  Determination iSi 

Index -o7 


VII 


CHAPTER   I 
THE  BACKGROUND  OF  GENETICS 

Genetics,  or  the  experimental  study  of  heredity,  was 
an  outgrowth  of  the  study  of  evolution.  A  ver>'  ])ricf 
survey  of  the  subject  of  evolution  before  1900  will  serve, 
therefore,  to  provide  a  background  for  the  material  of 
the  present  text,  by  depicting  something  of  what  was 
going  on  in  the  minds  of  biologists  at  the  time  that 
genetics  had  its  birth.  It  will  also  be  useful  to  have  before 
us  some  of  the  ideas  of  evolution  as  a  means  of  suggesting 
a  wider  application  of  the  principles  of  genetics  that  are 
to  be  taken  up. 

Nothing  need  be  said  here  of  that  phase  of  the  evolu- 
tion enterprise  which  concerned  itself  with  convincing 
a  doubting  public  of  the  mere  fact  of  evolution.  The 
other  phase,  involving  the  presentation  of  ex})lanations 
of  the  evolutionary  process,  will  be  sketched  briefly. 

The  vague  ideas  of  evolution  that  occupied  the  minds 
of  men  during  the  earKer  history  of  biology  and  the  fan- 
tastic speculative  explanations  that  were  proposed  have 
Httle  more  than  historical  interest  for  us  today.  These 
explanations  were  based  upon  meditation  rather  than 
investigation,  so  that  they  resembled  philosophy  rather 
than  science. 

Around  the  latter  part  of  the  eighteenth  ccntur\', 
certain  men  (notably  Erasmus  Darwin,  Goetiik,  and 
St.  Hilaire)  developed  more  accurate  notions  of  evolu- 
tion, based  in  good  part  upon  their  own  observations, 

I 

nOFBRTY  LIBRARY 

n.  C.  State  CoUem 


2  Outline  of  Genetics 

and  proposed  simple  explanations  of  the  process.  These 
explanations  called  upon  the  direct  influence  of  the  envi- 
ronment, but,  since  little  effort  was  made  to  analyze  the 
process  any  further  than  this,  these  theories  have  little 
value  for  us. 

The  first  author  to  provide  any  thoroughgoing  expla- 
nation of  evolution  was  Lamarck,  and  his  theory  of 
Use  and  disuse  (1801)  still  commands  the  attention  of 
biologists.  According  to  Lamarck,  the  environment 
was  important,  not  as  a  direct  cause  of  evolution,  but 
merely  as  the  occasion  for  evolutionary  change.  When 
an  animal  came  to  live  under  changed  environmental 
conditions,  possibly  through  migration,  it  encountered 
certain  new  needs.  These  new  needs  stimulated  in  the 
animal  the  desire  to  satisfy  the  needs.  Following  this, 
the  animal  made  a  conscious  effort  to  satisfy  the  needs, 
and  in  this  effort  succeeded  in  exercising  certain  of  its 
organs  more  than  before.  This  exercise  resulted  in  the 
development  of  the  part  exercised.  At  this  point 
Lamarck  introduces  his  basic  assumption  to  the  eft'ect 
that  acquired  characters  are  inherited.  Whatever  gain 
is  made  in  developing  an  organ  through  exercise,  is  passed 
on  to  the  progeny.  The  progeny,  living  under  the  same 
environmental  conditions  and  actuated  by  the  same 
motives,  will  make  some  further  gain  in  the  development 
of  the  organ  in  question,  and  in  this  cumulative  manner 
the  organ  will  eventually  be  developed  to  such  an  extent 
that  a  new  species  may  be  said  to  have  originated. 

The  classic  example,  which  seems  rather  absurd  in 
itself,  but  serves  to  illustrate  Lamarck's  ideas,  runs  as 
follows.  The  horselike  ancestors  of  the  giraffe  come  to 
live  in  a  new  and  arid  environment,  such  that  the  only 


The  Background  of  Genetics  3 

substantial  forage  is  provided  by  the  leaves  of  occasional 
trees.  These  animals  need  to  reach  up  to  the  leaves,  and 
therefore  desire  to  do  so.  Through  a  conscious  ejjorl 
to  stretch  up  to  the  leaves,  their  necks  and  limbs  are 
exercised  in  such  a  way  as  to  lengthen  them.  The  small 
gain  in  length,  possibly  only  an  inch  or  less,  made  during 
the  lifetime  of  the  individual  is  passed  on  to  the  progeny, 
who  are  successful  in  adding  another  inch.  The  final 
result  is  the  giraffe.  This  evolutionary  scheme  works 
also  in  the  reverse  direction  as  the  result  of  degeneration 
through  disuse. 

Absurd  as  some  of  Lamarck's  illustrations  may  seem, 
he  has  really  provided  the  elements  of  a  complete  and 
not  unlikely  explanation  of  evolution.  The  major  objec- 
tion lay  in  his  assumption  of  the  inheritance  of  acquired 
characters.  Practically  all  of  the  earlier  expcrmients 
on  this  problem  seemed  to  demonstrate  that  inheritance 
of  acquired  characters  is  impossible,  and  it  was  for  this 
reason  that  the  majority  of  biologists  discarded 
Lamarck's  theory.  Another  objection  lay  in  the  fact 
that  conscious  effort  was  hardly  to  be  expected  among 
plants.  Lamarck  of  course  recognized  this  obvious 
difficulty,  and  revised  his  theory  in  the  case  of  plants, 
where  he  claimed  the  changes  were  brought  about 
through  the  direct  effects  of  the  environment,  citing  such 
things  as  soil,  temperature,  moisture,  and  mechanical 
pressure. 

The  next  great  explanation  was  presented  in  Charles 
Darwin's  Origin  of  species  in  1859.  Darwin  called 
attention  to  the  geometric  ratio  of  increase  among  living 
organisms,  and  reiterated  the  doctrine  of  ALvlthus  to 
the  effect  that  any  population  tends  to  increase  more 


4  Outline  of  Genetics 

rapidly  than  the  means  of  subsistence.  This  increase 
tends  to  set  up  a  severe  struggle  for  existence  or  compe- 
tition, as  the  result  of  which  an  equilibrium  of  species 
is  established,  with  approximately  the  same  number  of 
individuals  of  a  given  species  surviving  year  after  year 
in  any  given  locality.  Darwin  next  points  out  the 
universality  of  variation  among  living  organisms,  such 
that  no  two  individuals  of  any  species  are  ever  absolutely 
identical.  As  for  the  cause  of  variation,  no  explanation 
is  provided,  but  the  nature  of  variation  is  rather  clearly 
outlined.  Those  variations  which  are  important  in  the 
evolutionary  process  are  characterized  as  quantitative, 
continuous,  and  fluctuating.  By  quantitative  it  is 
meant  that  the  variations  are  differences  in  the  degree 
of  development  of  some  part  or  feature  of  the  organism. 
When  it  is  said  that  the  variations  are  continuous,  the 
implication  is  that  further  variations  will  take  place  in  the 
same  direction  as  the  variations  that  have  taken  place 
in  the  preceding  generations.  The  term  ''fluctuating" 
indicates  that  reverse  variations  will  take  place  as 
freely  as  do  the  progressive  variations.  According  to 
Darwin,  variation  of  this  type  is  going  on  in  all  organ- 
isms. Since  this  is  true,  and  since  a  severe  struggle 
for  existence  is  taking  place,  it  is  impossible  to  escape 
the  conclusion  that  it  is  the  ''fittest"  that  survive.  If 
a  given  species  is  represented  in  a  certain  locality  by  a 
thousand  young  individuals,  no  two  of  which  are  abso- 
lutely alike,  and  if  there  is  only  enough  room  or  only 
enough  subsistence  for  one  hundred  of  them  ever  to 
reach  maturity,  it  must  be  true  that,  in  general,  it  will 
be  those  that  are  the  best  adapted  to  cope  with  the 
conditions  of  the  environment  that  are  the  ones  to  sur- 


The  Background  of  Genetics  5 

vive,  while  the  rest  perish  in  the  struggle.  The  one 
hundred  survivors  are  ''litter"  than  were  the  others  of 
their  generation  because  they  happened  to  have  certain 
useful  organs  or  processes  somewhat  more  fully  de\'el- 
oped.  In  the  following  generation,  some  of  the  progeny 
will  have  the  organs  in  question  still  more  fully  de\'eloped 
than  did  their  parents,  while  on  the  other  hand  there 
will  also  be  some  that  have  them  less  fully  developed. 
The  former  group  will  again  be  chosen  by  nature  to  sur- 
vive and  perpetuate  the  species,  and  thus  progress  will 
be  made  in  the  direction  of  better  development  of  useful 
organs  until  a  degree  of  development  has  been  attained 
which  may  be  said  to  represent  a  new  species.  The  wa}^ 
in  which  nature  manipulates  these  quantitative,  con- 
tinuous variations  of  Darwin's  to  bring  about  this 
progressive  evolution  or  adaptation  can  be  \'isualized 
more  concretely  from  this  simple  diagram. 


(More  poorly 
adapted  forms 

perish  in 
competition) 


^         ^        6 


^         ^ 


Environment  -> 

(favors 

variations  in  this 

direction) 


^ 


8 


The  numerous  objections  to  Darwin's  theory  cannot 
be  discussed  here;  suffice  it  to  say  that  these  objections 
were  directed  mainly  at  the  adequacy  of  the  Darwinian 
variations  in  accounting  for  the  results  of  evolution 
rather  than  at  the  idea  of  natural  selection.  The  ''sur- 
vival of  the  fittest"  is  a  rather  generally  accepted  idea. 
The   question   whether    the   Darwinian   variations   are 


6  Outline  of  Genetics 

adequate  can  be  considered  more  critically  a  little  later 
in  the  light  of  the  more  recent  knowledge  of  inheritance. 

The  third  great  explanation  of  evolution  was  offered 
by  De  Vries  in  1900.  This  author  was  the  first  to  base 
his  conclusions  on  the  results  of  his  own  experimental 
breeding,  rather  than  merely  on  the  extensive  observa- 
tion of  plants  and  animals  in  nature.  Judging  from  the 
behavior  shown  by  Oenothera  Lamar ckiana  (American 
evening  primrose)  during  the  course  of  the  ten  or  more 
generations  that  it  grew  in  his  garden,  De  Vries  con- 
cluded that  the  real  basis  of  evolution  lay  in  the  phenom- 
enon of  mutation.  In  addition  to  its  ''normal"  pro- 
geny, 0.  Lamarckiana  produced  in  small  numbers  cer- 
tain distinctly  new  types,  the  mutants.  The  type  of 
variation  involved  in  mutation  was  distinctly  different 
from  the  Darwinian,  being  qualitative,  discontinuous, 
and  constant.  It  was  readily  seen  that  the  mutants 
involved  qualitative  changes  from  the  parent,  inasmuch 
as  entirely  new  characters  were  shown,  rather  than  merely 
the  quantitatively  greater  or  lesser  development  of  cer- 
tain of  the  parental  characteristics.  It  was  equally 
plain  that  mutation  was  discontinuous,  the  direction 
and  nature  of  mutations  being  entirely  unrelated  to  any 
of  the  mutations  that  had  taken  place  in  the  past.  And 
finally,  the  mutants  were  strikingly  constant,  breeding 
true  to  their  own  characteristics  rather  than  reverting 
in  later  generations  to  the  original  parental  type.  These 
mutational  changes  that  De  Vries  studied  intensively 
in  Oenothera  were  later  identified  in  other  species  as  well. 

The  part  that  mutation  may  play  in  evolution  is 
suggested  by  a  consideration  of  the  characteristics  of  the 
mutants.     Probably  the  majority  of  the  mutants  differ 


The  Background  of  Genetics  7 

from  the  parent-form  in  such  ways  that  thc>'  might  well 
be  called  degenerates;  the  new  characteristics  shown 
serve  to  adapt  the  mutant  more  poorly  to  the  environ- 
ment than  the  parent  was  adapted.  Other  mutants 
may  show  changes  only  of  an  unimportant  type,  so  that 
they  are  neither  better  nor  more  poorly  adai)ted  than 
was  the  parent.  A  few  of  the  mutants  (according  to 
De  Vries)  may  show  such  characteristics  as  to  be 
better  adapted  to  the  environment  than  was  the  parent- 
form.  Upon  this  miscellaneous  mass  of  mutants  natural 
selection  immediately  comes  to  play,  quickly  eliminating 
the  poorly  adapted  types  and  preserving  the  good. 
Thus  De  Vries  holds  with  Darwin  in  invoking  natural 
selection,  but  the  t^pe  of  variations  involved  is  dis- 
tinctly different.  According  to  Darwin,  natural  selection 
serves  gradually  to  build  up  a  new  species;  according 
to  De  Vries,  numerous  new  species  are  born  full  Hedged, 
and  natural  selection  merely  decides  which  of  them  shall 
survive.  Objections  to  the  De  Vriesian  theory  will  be 
mentioned  later  in  this  text,  in  the  light  of  some  of  the 
rather  recent  work  in  genetics. 

In  addition  to  these  three  great  explanations  of  evolu- 
tion, there  are  a  few  others  that  should  be  considered 
briefly.  A  number  of  authors  (notably  David  Starr 
Jordan  in  this  country)  have  attached  primary  impor- 
tance to  the  principle  of  "'isolation"  in  evolution.  A 
few  individuals  of  a  species  may  migrate  successfully 
to  a  new  locality  which  is,  or  may  subsequently  become, 
sufficiently  isolated,  by  geographic  barriers  of  one  t}pe 
or  another,  from  the  original  locality  that  no  extensive 
return  migration  can  take  place.  The  result  is  that  a 
new  colony  is  established  which  is  sufficiently  isolated 


g  Outline  of  Genetics 

from  the  parent  realm  that  free  crossing  between  the 
colonists  and  the  stay-at-homes  does  not  take  place. 
Any  chance  variations  that  may  have  existed  in  the  few 
original   emigrants   will   now   have    an  opportunity   to 
pe^etuate  themselves  instead  of  being  ''swamped  out" 
through  free  crossing  with  the  other  members  of  the 
parent  species,  as  would  have  happened  if  there   had 
been  no  isolation.     Thus  the  various  populations  of  a 
species  that  we  see  today  somewhat  isolated  from  one 
another  have  had  a  chance  to  express  and  later  augment 
chance  differences  to  such  a  degree  that  we  may  now 
regard  them  as  different  varieties.     With  time  the  diver- 
gence  of   characteristics  between   the   isolated   groups 
will  become  still  greater. 

The  isolation  theory,  as  outlined  above,  does  not  by 
itself  provide  a  ''complete"  explanation  of  evolution. 
It  is  best  to  be  regarded  as  a  sort  of  a  coroUary  to  the 
Darwinian  or  DeVriesian  schemes.    In   either   event, 
it  is  the  principle  of  natural  selection  that  brings  about 
progressive  or  adaptive  evolution,  while  isolation  either 
serves  to  multiply  species  "on  the  same  level,"  through 
giving   chance   variations    an    opportunity    to    express 
themselves  and  become  augmented,  or  else  it  serves  to 
enlarge  the  scope  of  natural  selection  by  thrusting  repre- 
sentatives of  the  species  into  a  somewhat  new  environ- 
ment or  by  presenting  natural  selection  with  a  slightly 
new  population  from  which  to  make  the  choices. 

Another  matter  that  should  be  mentioned  is  ortho- 
genesis. Orthogenesis  may  be  regarded  either  as  an 
"explanation"  of  evolution  or  merely  as  the  name  of  a 
phenomenon.  There  is  considerable  evidence  support- 
ing the  beUef  that  the  variations  (or  at  least  many  of 


The  Background  of  Genetics  9 

Ihem)  that  take  place  in  li\ijig  organisms  arc  determinate^ 
taking  place  along  predetermined  lines,  in  a  predictable 
direction,  rather  than  indeterminate,  with  an  equal  chance 
of  their  taking  place  in  any  direction,  as  the  other  theories 
would  have  it.  For  example,  if  a  variant  is  slightly 
dwarfed  as  compared  with  the  parent  type,  there  will 
be  among  the  descendants  of  this  variant  a  further  varia- 
tion involving  greater  dwarfedness,  and  subsequently 
more  changes  will  take  place  all  involving  further  steps 
along  this  same  predetermined  line.  The  direction  of 
the  variations  is  not  necessarily  related  to  any  environ- 
mental demand. 

A  few  authors,  regarding  orthogenesis  as  an  "explana- 
tion" of  evolution,  have  visualized  an  intrinsic  ''force" 
in  the  organism  which  guides  the  variations.  Since  this 
view  has  a  vitalistic  flavor,  it  is  not  popular  among  scien- 
tific men.  More  often  it  is  simply  recognized  that  a 
certain  amount  of  variation  of  this  sort  does  take  place, 
and  orthogenesis  is  the  name  given  to  the  phenomenon, 
various  quite  materialistic  explanations  having  been 
proposed  to  account  for  it. 

"Evolution  through  hybridization"  is  a  theory  that 
was  suggested  by  Weismann  some  decades  ago,  and  has 
recently  been  developed  and  championed  by  Lotsy. 
It  is  a  fact  well  known  among  biologists  that  crossing 
two  distinct  types  may  result,  in  the  second  hybrid 
generation,  in  a  few  new  and  pure-breeding  forms,  some- 
what different  from  anything  that  had  previously 
existed.  Lotsy  has  shown  by  experiment  that  when 
such  new  forms  (from  Antirrhinum  crosses)  are  returned 
to  grow  under  natural  conditions,  nature  will  select 
some  of  the  types  to  survive,  but  will  quickly  eliminate 


lo  Outline  of  Genetics 

the  others.  While  there  is  little  question  that  natural 
hybridization  takes  place  and  may  be  a  real  factor  in 
producing  new  varieties,  at  the  same  time  this  theory 
is  not  satisfactory  as  a  ''complete"  explanation  of  evo- 
lution. It  seems  rather  obvious  that,  although  hybridi- 
zation can  multiply  variations  through  crossing  forms 
that  are  already  different  from  each  other,  it  can  never 
account  for  the  "original"  differences. 

In  considering  the  relative  merits  of  these  different 
explanations,  there  are  three  things  that  it  is  useful 
to  bear  in  mind.  First,  it  is  certainly  not  necessary  to 
subscribe  to  a  belief  in  any  one  of  the  theories  to  the 
complete  exclusion  of  the  others.  It  is  quite  possible 
that  every  one  of  them  may  be  a  factor  in  evolution, 
and  altogether  probable  that  no  one  of  them  by  itself 
can  adequately  account  for  all  of  the  evolutionary  change 
that  has  taken  place. 

Second,  it  is  not  advisable  to  contrast  these  explana- 
tions as  though  they  were  coordinate  units.  The  "prob- 
lem of  evolution"  is  not  a  single  problem,  but  a  complex 
of  numerous  ones,  and  any  proposed  explanation  of 
evolution  is  confronted  by  the  necessity  of  answering 
several  distinct  questions.  The  conspicuous  questions 
to  be  answered  are: 

1.  What  is  the  cause  of  variation? 

2.  What  is  the  nature  of  the  variations  that  are 
important  in  evolution? 

3 .  How  may  variations  be  perpetuated  and  multiplied  ? 

4.  How  are  the  variations  manipulated  to  effect 
progressive  evolution? 

It  will  be  noticed  that  Lamarck  goes  farther  than 
any  other  author  in  answering  question  i.     Orthogenesis 


The  Background  of  Genetics  ii 

and  the  hybridization  theory  provide  suggestions  on 
this  point,  but  the  suggestions  are  hardly  satisfactory. 
For  question  2  rather  distinct  answers  are  provided  by 
Lamarck,  Darwin,  De  Vries,  and  the  orthogenesis 
theory.  It  is  in  answering  question  3  that  the  isolation 
theory  and  the  hybridization  theory  have  their  chief 
value.  Question  4  is  indirectly  answered  in  one  way  by 
Lamarck,  and  indirectly  answered  in  another  way  in 
the  orthogenesis  theory,  while  all  the  other  theories 
plainly  call  upon  natural  selection  to  answer  this  ques- 
tion. If  the  theories  are  to  be  compared,  it  can  safely 
be  done  only  after  some  such  analysis  as  this. 

Third,  discussion  of  evolutionary  theories  usually 
leads  to  the  realization  that  more  exact  experimental 
evidence  is  needed  before  much  further  progress  can  be 
made  in  solving  the  problems  of  evolution.  Such  has 
been  the  actual  history  of  the  case,  for,  with  the  begin- 
ning of  the  twentieth  century,  the  study  of  evolution 
culminated  in,  and  became  diverted  into,  genetics,  the 
experimental  study  of  inheritance.  Of  course  genetics 
has  not  answered  all  of  the  questions  that  have  presented 
themselves  in  connection  with  evolution,  but  many  criti- 
cal and  suggestive  findings  have  been  made,  as  will  be 
seen  in  the  following  chapters;  and  unquestionably 
genetics  will  contribute  a  great  deal  more  in  the  next 
few  decades. 


CHAPTER  II 

THE  INHERITANCE  OF  ACQUIRED 
CHARACTERS 

At  the  basis  of  genetics  lies  the  fact  that  variation 
occurs  in  all  living  organisms.  It  is  possible  to  classify 
variations  in  a  number  of  different  ways.^  At  the  out- 
set it  is  important  to  realize  the  distinction  between  non- 
heritable  and  heritable  variations.  As  for  the  former, 
it  is  usually  evident  that  these  originate  as  responses  on 
the  part  of  the  organism  to  environmental  stimuli.  Ac- 
quired characters  of  this  sort,  however,  are  of  little  sig- 
nificance in  genetics,  inasmuch  as  they  are  not  passed  on 
from  parent  to  offspring.  It  is  the  heritable  variations 
that  provide  the  material  of  genetics;  and  the  origin  of 
these  is  a  matter  of  considerable  controversy.  For  the 
most  part,  they  are  ascribed  to  mutation,  meaning  that 
their  origin  is  sudden  and  spontaneous,  seemingly  unre- 
lated to  environmental  stimuli.  There  is  some  evidence, 
however,  which  suggests  that  heritable  variations  may 
originate  as  acquired  characters.  It  will  be  appropriate 
at  this  point  to  discuss  the  controversy  on  inheritance  of 
acquired  characters. 

The  idea  of  inheritance  of  acquired  characters  was 
first  clearly  developed  by  Lamarck  in  connection  with 
his  explanation  of  evolution,  the  so-called  theory  of 
*' appetency,"  or  the  effect  of  use  and  disuse.  Francis 
Galton,  in  1875,  was  one  of  the  first  to  express  skepti- 

*  A  serviceable  set  of  classifications  is  provided  by  Babcock  and 
Claussen  (i). 

12 


TJie  InJieritance  of  Acquired  Characters  13 

cism  in  regard  to  such  inheritance,  but  it  was  Weis- 
MANN  (17)  who  was  most  intluential  in  combating 
the  idea.  After  Weismann's  presentation  of  the  sit- 
uation, biologists  were  divided  into  two  camps  in 
reference  to  the  question:  (i)  neo-Lamarckians,  who 
affirmed  behef  in  inheritance  of  acquired  characters, 
and  (2)  neo-Darwinians,  who  denied  it.  Until  very 
recently,  at  least,  the  bulk  of  the  evidence  of  genetics 
has  served  to  refute  inheritance  of  acquired  charac- 
ters. 

Much  of  the  lack  of  agreement  in  this  controversy 
is  due  to  the  dehnition  of  an  acquired  character.  It 
should  be  kept  in  mind  that  actual  characters  are  not 
inherited,  but  only  the  determiners,  which  regulate  the 
way  in  which  the  organism  reacts  to  its  enviromncnt. 
For  example,  when  it  is  said  that  a  child  inherits  its 
father's  nose,  the  statement  is  not  meant  to  be  literally 
true;  it  is  meant  that  just  as  there  was  something  in  the 
body  of  the  father  that  was  responsible  for  the  develop- 
ment of  a  particular  type  of  nose,  so  there  was  a  similar 
something  in  the  child's  body  that  developed  a  similar 
result.  It  is  merely  a  matter  of  convenience  to  speak 
of  the  inheritance  of  characters. 

Weismann  defined  an  acquired  character  as  ''any 
somatic  modification  that  does  not  have  its  origin  in  the 
germ  plasm."  This  definition  is  not  always  easy  to 
apply.  Examples  of  acquired  characters  in  the  Weis- 
mann sense  are  mutilations,  results  of  function  (as  in 
the  use  or  disuse  of  certain  organs),  many  diseases  that 
affect  the  bodily  mechanism,  and,  to  use  a  rather  vague 
expression,  effects  of  environment.  Weismann  gave 
three  reasons  for  rejecting  the  belief  in  inheritance  of 


14  Outline  of  Genetics 

such  characters:  (i)  there  is  no  known  mechanism  by 
which  somatic  characters  may  be  transferred  to  the  germ 
plasm;  (2)  the  evidence  that  such  a  transfer  does  occur 
is  inconclusive  and  unsatisfactory;  and  (3)  the  theory 
of  the  continuity  of  the  germ  plasm  is  sufficient  to  account 
for  the  facts  of  heredity. 

When  Weismann  says  that  there  is  no  known  mech- 
anism by  which  somatic  characters  can  be  transferred  to 
the  germ  plasm,  to  him  it  is  equivalent  to  saying  that  it 
is  hard  to  see  how  the  water  that  has  gone  over  the  dam 
can  return  and  affect  the  flow  of  the  water  upstream. 
He  assumes,  of  course,  that  the  genu  plasm  is  isolated 
from  the  somatoplasm  very  early  in  the  development 
of  the  fertilized  egg  into  an  individual,  and  that  w^hen  it 
is  isolated  it  takes  no  active  part  in  the  history  of  the 
body  (see  fig.  i).  The  somatoplasm  is  thus  merely 
a  carrier  of  the  germ  plasm,  and  is  unable  to  affect 
the  character  of  it  any  more  than  a  rubber  hot- 
water  bag,  although  capable  of  assuming  a  variety  of 
shapes,  can  affect  the  character  of  the  water  it  contains 
(Walter  18). 

This  early  differentiation  of  germ  plasm  and  body 
plasm  has  been  demonstrated  rather  strikingly  in  several 
animals.  In  Ascaris  megacephala,  the  following  cy to- 
logical  situation  was  demonstrated  by  Boveri,  in  1903 
(DoNCASTER  7).  Following  the  first  division  of  the 
zygote,  the  two  daughter-cells  come  to  differ  from  each 
other  through  the  apparent  degeneration  of  some  of  the 
cell  constituents  in  one.  That  daughter  which  main- 
tains the  full  cell  equipment  of  the  zygote  thereby  per- 
petuates the  capacities  of  the  germ  plasm,  while  the  other 
daughter,  which  has  lost  certain  visible  cell  constituents, 


Fig.  I. — Diagnim  illustrating  Weismann's  theory  of  germinal  con- 
tinuity. Three  generations  are  represented,  with  cells  of  germ  plasm 
shaded,  and  those  of  body  plasm  unshaded;  germ  plasm  continuous  from 
generation  to  generation,  carried  over  from  parent  to  ofTsping  by  z\gote 
(Z);  impossible  for  body  plasm  to  perpetuate  itself  into  a  second 
generation. 


1 6  Outline  of  Genetics 

starts  a  line  of  purely  body  plasm  cells.  A  similar 
differentiation  occurs  between  the  granddaughter-cells 
from  the  fully  equipped  daughter,  and  so  on  for  two  more 
divisions,  so  that  finally  there  are  fifteen  body  plasm 
cells  and  but  one  germ  plasm  cell;  the  germ  cells  of 
the  adult  individual  can  all  be  traced  to  this  single 
initial. 

A  somewhat  similar  program  has  been  traced  in  Chry- 
somelid  beetles,  where,  after  numerous  segmentation 
divisions,  some  of  the  nuclei  associate  with  certain  gran- 
ules, and  it  is  these  nuclei  that  start  the  germ  plasm. 
Hegner  (Doncaster  7)  has  succeeded  in  artificially 
destroying  these  granules  by  means  of  a  hot  needle, 
thus  producing  embr}'os  without  germ  cells. 

Equally  striking  situations  have  been  demonstrated 
in  other  animals  as  well,  but  nothing  of  the  sort  has 
ever  been  found  in  plants.  Germ  cells  in  plants  are 
formed  from  h^-podermal  and  even  epidermal  cells, 
which,  during  earlier  ontogeny,  are  apparently  identical 
with  other  somatic  tissues.  Here  there  is  surely  no  dis- 
tinct germ  plasm,  isolated  from  body  plasm  and  insulated 
within  it  from  environmental  influences.  In  fact,  there 
are  cases  in  which  ''adventitious"  germ  cells  have  been 
seen  to  form  from  tissues  which  normally  are  quite  as 
somatic  as  any  plant  tissue  could  be.  In  this  connection, 
it  is  worth  mentioning  that  Bateson  (2)  suspects  plants, 
as  genetic  machines,  differ  fundamentally  from  animals; 
this  idea  being  suggested  to  him  in  good  part  by  the 
fact  that  ''in  the  animal  the  rudiments  of  gametes  are 
often  visibly  separated  at  an  early  embryonic  stage, 
whereas  in  the  plant  they  are  given  off  from  persistent 
growing  points." 


The  Inheritance  of  Acquired  Characters  17 

A  general  theoretical  objection  to  Weismanx's  view 
is  that  every  organism  is  a  physiological  as  well  as  a 
morphological  unity,  and  that  cells  completely  insulated 
in  such  a  unity  would  be  impossible.  Cytologists  also 
have  come  to  believe  that  there  are  proto})lasmic  con- 
nections between  adjacent  cells  in  practically  all  plant 
tissues,  and,  in  general,  physiolog}'  tends  to  confirm 
this.  Such  suggestions  voice  a  growing  belief  that  the 
body  plasm  can  affect  the  germ  plasm. 

The  reply  of  the  Weismannians  is  that  even  though 
somatoplasm  might  affect  germ  plasm  in  this  general 
physiological  way,  this  is  a  very  different  thing  from  the 
inheritance  of  some  definite  acquired  character.  To  be 
inherited  such  a  character  would  have  to  be  exactly 
redeveloped  in  the  germ  plasm,  and  the  intlucnce  referred 
to  cannot  be  so  specific  as  that.  This,  of  course,  is  a 
theoretical  answer,  and  the  question  can  only  be  decided 
by  experimental  work.  A  theoretical  rejoinder  to  this 
answer  may  be  suggested.  It  is  like  the  voice  in  a  tele- 
phone transmitter,  which  starts  vibrations  that  make 
the  receiver  repeat  the  voice.  (Something  more  delinite 
on  this  matter  will  be  considered  a  little  later.)  Before 
arriving  at  anything  like  a  conclusion  on  this  matter, 
it  will  be  necessary  to  consider  some  of  the  claimed  cases 
of  inheritance  of  acquired  characters. 

I.  Mutilations. — Most  of  the  evidence  under  this 
head  is  in  relation  to  animals.  It  is  a  matter  of  common 
experience  that  mutilations  are  not  inherited  in  man 
and  the  domesticated  animals.  A  few  quotations  from 
Walter  (18)  suggest  the  situation: 

"It  is  fortunate  that  the  sons  of  warriors  do  not 
inherit  their  fathers'  honorable  scars  of  battle,  else  we 


1 8  Outline  of  Genetics 

would   now  be  a   race  of  cripples The   feet   of 

Chinese  women  of  certain  classes  have  for  centuries 
been  mutilated  into  deformity  by  bandages  without  the 
mutilation  in  any  way  becoming  an  inherited  char- 
acter  The  progressive  degeneration  or  crippling 

of  the  little  toe  in  man  has  been  explained  as  the  inherit- 
ance of  the  cramping  effect  of  shoes  upon  generations  of 
shoe-wearers;  but  Wiedersheim  has  pointed  out  that 
Egyptian  mummies  show  the ,  same  crippling  of  the 
little  toe,  and  no  ancient  Egyptian  could  be  accused  of 
wearing  shoes,  or  of  having  shoe-wearing  ancestors." 

Sheep  and  horses  with  docked  tails,  as  well  as  dogs 
with  cropped  ears,  never  produce  young  having  the 
parental  deformity.  Weismann's  early  experiments 
with  mice,  later  verified  by  other  investigators,  give 
additional  evidence  that  mutilations  are  not  inherited. 
He  bred  mice  whose  tails  had  been  cut  off  short  at  birth, 
and  continued  this  performance  through  twenty-two 
generations,  with  absolutely  no  eft'ect  on  tail  length. 

Very  little  serious  consideration  has  been  given  to  the 
possibility  of  inheritance  of  mutilations  in  plants.  Cut- 
tings for  propagation  are  usually  trimmed  to  prevent 
excessive  transpiration,  but  no  one  ever  expects  to  find 
this  mutilation  perpetuated,  even  in  the  plant  developed 
from  the  cutting,  much  less  in  the  next  generation  devel- 
oped from  seed.  In  fact,  since  we  have  begun  to  learn 
of  the  remarkable  powers  of  regeneration  possessed  by 
plants  and  animals,  we  would  not  expect  the  inheritance 
of  mutilations. 

There  is  one  bit  of  work  that  should  be  mentioned  in  this  con- 
nection. Blaeinghem  (3)  claims  to  have  procured  from  a  single 
injured  individual  a  Hne  of  maize  plants  that  show  a  varying  per- 


The  Inlientance  of  Acquired  Characters  19 

centage  of  double  and  sometimes  triple  grains.  The  author  calls 
this  a  typical  case  of  inheritance  of  acquired  characters,  but  Ameri- 
can investigators  have  hesitated  to  accept  this  interpretation  of 
the  phenomenon.  Characters  of  practically  the  same  sort  have 
been  observed  to  originate  in  other  cases  in  corn  not  known  to 
have  been  injured  in  any  way. 

2.  Effects  of  use  and  disuse. — Inheritance  of 
the  effects  of  use  and  disuse  lay  at  the  foundation  of 
Lamarck's  theory  of  evolution.  Weismann  was  suc- 
cessful in  discrediting  this  belief  by  explaining  on  some 
other  basis  practically  all  of  the  supposed  examples  of 
this  phenomenon  that  had  been  advanced.  In  plants, 
of  course,  it  would  be  hard  to  find  anything  exactly 
analogous  to  the  use  and  disuse  of  parts  in  animals; 
Lamarck  himself  did  not  attempt  to  apply  quite  the 
same  theory  to  the  plant  kingdom. 

One  fact,  however,  is  a  common  experience  of  botanists. 
Functionless  organs  gradually  become  aborted,  become  mere 
vestiges  or  even  suppressed  entirely.  For  example,  a  study  of  the 
organogeny  of  flowers  shows  that  when  a  lloral  member  is  belated 
in  its  development  it  is  destined  sooner  or  later  not  to  appear  at 
all.  The  following  theoretical  Weismannian  (or  Darwinian) 
explanation  of  this  situation  is  suggested.  A  given  species  has  a 
given  nutritive  capacity;  the  less  it  draws  upon  its  nutritive 
capital  for  the  development  of  one  organ  the  more  it  has  available 
to  expend  on  the  development  of  other  organs.  When  an  organ 
becomes  functionless  it  no  longer  has  any  survival  value;  survival 
is  then  dependent  upon  the  relative  develoi^ment  of  the  other 
organs.  Through  "variation"  certain  individuals  develop  the 
functionless  organ  less  than  usual  and  therefore  develop  the  other 
organs  more  than  usual.  Under  the  new  conditions  these  individ- 
uals will  survive  and  the  others  will  be  eliminated.  Gradually 
abortion  of  functionless  organs  would  lake  [)lace  in  this  waw 
One  would  expect  that  the  rate  of  change  would  be  roughl>-  pro- 


20  Outline  of  Genetics 

pyortional  to  the  size  of  the  organ  involved,  and  that  any  retrogres- 
sive evolution  of  this  sort  would  be  slower  than  progressive  evolu- 
tion. 

3.  Diseases. — Roughly  speaking,  diseases  are  either 
the  results  of  infection  by  bacteria  or  fungi  or  some  inher- 
ent organic  weakness.  Since  the  latter  condition  is 
chiefly  serious  only  in  inviting  attacks  by  bacteria  and 
fungi,  we  are  concerned  chiefly  with  diseases  caused  by 
these  pathogenic  forms.  Realizing  this,  true  inheritance 
of  disease  seems  to  be  an  impossibility,  for  if  the  parasite 
enters  the  germ  cell  it  is  practically  sure  to  destroy  it, 
and  there  will  be  no  progeny.  It  is  true  that  in  many 
cases  progeny  are  born  diseased,  but  this  is  due  to  rein- 
fection of  the  young  embryo  from  the  body  of  the  mother. 
Many  examples  of  this  phenomenon  are  available  in  man 
and  other  mammals.  In  plants,  also,  diseases  (e.g., 
smut)  are  sometimes  passed  on  by  means  of  spores  car- 
ried upon  or  even  within  the  seeds.  Such  a  thing,  how- 
ever, can  in  no  sense  be  spoken  of  as  inheritance,  since 
it  always  involves  a  reinfection. 

In  one  respect,  however,  one  may  speak  of  disease 
inheritance.  Breeding  experiments  have  shown  that 
predisposition  to  disease  and  disease  resistance,  com- 
monly called  susceptibility  and  immunity,  are  inherited. 
In  practically  all  cases,  these  characteristics  are  evidently 
of  germinal  origin,  having  been  hereditary  in  the  begin- 
ning rather  than  acquired.  Such  cases,  of  course,  have 
no  bearing  on  the  present  problem.  There  remain  a 
few  instances,  however,  that  rather  suggest  the  inherit- 
ance of  acquired  characters. 

GuYER  and  Smith  (10),  by  inoculating  female  rabbits 
either  with  typhoid  vaccine  or  with  the  hving  bacilli. 


TJie  InJieritauce  of  Acquired  Characters  21 

have  succeeded  not  only  in  building  \i\)  a  high  resistance 
to  typhoid  in  these  female  rabbits  themselves,  but  in 
securing  from  them  progeny  with  a  high  resistance.  In 
fact,  rabbits  of  the  third  generation  have  still  shown  a 
high  immunity  which  could  have  come  only  from  their 
grandmothers.  The  likelihood,  however,  is  that  this 
immunity  is  not  passed  on  through  the  germ  cell  itself, 
but  is  "reacquired"  by  offspring  while  in  utero,  and 
nourished  by  the  blood  stream  of  the  mother.  This,  of 
course,  would  again  be  merely  a  case  of  ''transmission" 
rather  than  true  inheritance.  The  passing  on  of  such 
an  acquired  immunity  from  the  male  parent  to  the  pro- 
geny would  constitute  a  convincing  demonstration  of 
inheritance  of  acquired  characters,  but  such  a  demon- 
stration has  not  as  yet  been  made. 

.  It  is  suspected  that  a  situation  similar  to  the  foregoing  exists 
also  in  man.  Racial  immunity  is  believed  by  some  medical  men 
to  have  been  built  up  not  only  through  a  "natural  selection"  of 
immune  types,  but  from  the  passing  on  from  mother  to  offspring 
of  acquired  immunity. 

There  is  one  fairly  well  known  case  of  this  sort  in  the 
plant  kingdom.  Bolley  (4)  claims  that  he  can  get  a 
resistant  strain  of  flax  from  almost  any  known  variety. 
According  to  him,  the  resisting  ability  increases  from 
generation  to  generation,  if  the  crop  is  constantly  sub- 
jected to  disease  attack.  He  took  a  ])ure-pedigreed 
strain  of  flax  which  had  come  original  1>'  from  a  single 
non-resisting  seed.  This  was  planted  in  slightly  ''sick" 
soil,  that  is,  soil  infected  with  the  wilt-i)r()(lucing  organ- 
ism. Most  of  the  individuals  died,  1)ut  ''a  few  scrubs" 
survived.  He  then  planted  seeds  from  these  in  slightl\- 
"sicker"  soil  than  before,  and  thus,  by  gradually  work- 


22  Outline  of  Genetics 

ing  his  crop  into  sicker  and  sicker  soil  in  the  later  genera- 
tions, he  finally  obtained  a  fully  resistant  strain  from  the 
pure  non-resistant  strain  with  which  he  started.  Such  a 
strain,  he  says,  will  not  lose  its  resistance  if  planted 
progressively  in  more  infected  soils.  He  gives  the  fol- 
lowing theoretical  explanation  of  his  results: 

''Either  (i)  the  so-called  unit  character  of  resistance 
was  present  in  undeveloped  form  and  becomes  stronger 
from  year  to  year  under  conditions  of  disease;  or  (2)  there 
never  was  any  character  present  which  is  entitled  to  be 
called  a  unit  character,  but  it  began  to  develop  the 
first  year  the  parent  plant  came  in  contact  with  the 
disease,  and  the  protoplasmic  nature  of  the  ancestors  of 
the  plants  which  we  now  have  has  been  such  that  they 
accumulated  more  and  more  the  resisting  power  from 
year  to  year,  just  as  they  had  opportunity  to  develop 
resistance  against  a  constantly  acting  factor  of  disease, 
which,  when  too  powerful,  acts  as  an  eliminating  factor." 

BoLLEY  inclines  to  the  second  alternative.  This 
general  conception  seems  to  explain  why  home-grown 
seed  is  regularly  more  resistant  than  seed  from  the  same 
variety  which  has  had  a  vacation  away  from  home  for 
several  years.  It  has  kept  in  training  like  a  football 
player.  Bolley  says  that  if  these  conclusions  are  cor- 
rect, there  are  probably  no  unit  characters  which  are  not 
fluctuating,  and  there  are  no  fluctuating  characters 
which  may  not  readily  be  fijced. 

These  results  are  striking  enough,  but  their  signifi- 
cance depends  entirely  upon  the  purity  of  the  strains 
which  were  used  originally,  and  also  upon  the  preserva- 
tion of  purity  during  the  experiment.  Bolley's  phrase 
"elimination  factor,"  which  he  uses  repeatedly,  might 


The  Inherilance  of  Acquired  Characters  23 

be  taken  to  suggest  selection  from  an  impure  strain.  If 
his  conception  is  true,  it  could  be  demonstrated  by  de- 
veloping a  large  majority  of  resistant  indiA'iduals  among 
the  non-resistant  j)lants  which  were  first  subjected  to 
disease  attack,  rather  than  merely  ''a  few  scrubs."  The 
results  as  they  stand  could  probably  be  interpreted  as 
due  to  the  selection  of  a  few  resistant  individuals  from 
an  impure  strain. 

From  the  foregoing  cases,  it  becomes  rather  evident 
that,  so  far  as  mutilations,  effects  of  use  and  disuse,  and 
diseases  are  concerned,  inheritance  of  acquired  characters 
has  not  as  yet  been  satisfactorily  demonstrated,  either 
in  the  plant  or  animal  kingdom.  One  more  category 
of  cases,  however,  remains  to  be  considered. 

4.  Effects  of  environment. — This  heading  is  suffi- 
ciently inclusive  to  include  a  number  of  types  of  cases. 

It  has  now  been  some  years  since  Castle  (6)  per- 
formed his  classic  experiment  on  guinea  pigs.  Animals 
w^ith  white  coats  will  have  only  white-coated  progeny, 
while  a  pair  with  black  coats,  provided  both  male  and 
female  come  from  a  pure  stock,  will  have  only  black- 
coated  progeny.  Using  only  animals  from  pure  stock, 
Castle  removed  the  ovaries  from  a  white-coated  female 
and  transplanted  them  into  the  body  of  a  black-coated 
female.  The  mating  between  this  black-coated  '^ foster 
mother"  and  a  white-coated  male  resulted  in  a  progeny 
all  of  which  had  white  coats.  Evidently  it  was  the  germ 
cells  alone  that  were  effective  in  determining  the  char- 
acter of  the  progeny.  The  decisive  results  of  this  experi- 
ment were  very  influential  in  refuting  the  concept  of 
inheritance  of  acquired  characters.  At  the  same  time 
it  must  be  borne  in  mind  that,  whereas  such  a  superficial 


24  Outline  of  Genetics 

character  as  coat  color  might  not  respond  to  artificial 
manipulation  of  the  germ  cells,  it  is  still  possible  that 
there  are  other  characters,  more  fundamentally  tied  up 
with  the  metabolism  of  the  organism,  that  could  be 
affected  by  such  treatment. 

Numerous  experiments  confirmed  these  findings  of 
Castle's,  but  there  was  one  field  of  investigation  from 
which  rather  contradictory  results  began  to  be  accumu- 
lated. The  numerous  studies  that  have  been  made  dur- 
ing the  last  few  years  on  inheritance  in  the  microorgan- 
isms have  been  ably  summarized  by  Jennings  (13). 
Here  there  appear  some  striking  indications  of  inheritance 
of  acquired  characters. 

''The  germinal  or  genotypic  constitution  in  most 
organisms  is  extremely  stable;  in  many  stocks  it  changes 
not  at  all,  so  far  as  observation  goes.  To  alter  it  by 
physical  or  chemical  agents  is  usually  to  kill  it.  In 
some  of  the  lowest  organisms — rhizopods,  bacteria,  some 
infusoria^ — it  changes  with  somewhat  greater  frequency, 
though  still  rarely.  The  nature  of  the  changes,  and 
whether  they  may  be  permanent,  or  must  after  genera- 
tions revert  to  the  original  condition,  is  in  some  dispute. 
In  these  same  organisms,  environmental  agents  may  pro- 
duce changes  persisting  through  many  generations  of 
uniparental  reproduction  and  even  through  biparental 
reproduction,  the  period  of  persistence  depending  partly 
on  the  number  of  generations  through  w^hich  the  pro- 
ducing agent  acted.  This  suggests  that  inherited  char- 
acters as  permanent  as  any  that  exist  might  in  time  be 
so  produced.  In  spite  of  important  differences  of  opinion 
among  investigators,  to  the  reviewer  the  facts  in  uni- 
parental reproduction  seem  to  point  more  toward  the 


pfOfOnT  LBRARf 

N.  C.  Stale  College 


The  Inlieritance  of  Acquired  Characters  25 

production  of  ev^oluliouary  change  by  the  action  of  the 
environment  on  the  germ  plasm  than  hy  any  of  the 
other  methods." 

This  behavior  on  the  part  of  some  of  the  lower  organ- 
isms, difficult  to  interpret  without  the  assumi)tion  of 
inheritance  of  acquired  characters,  fostered  the  following 
belief.  In  higher  animals,  where  germ  plasm  and  body 
plasm  are  sharply  differentiated,  inheritance  of  acquired 
characters  is  an  impossibility;  in  the  simpler  organisms, 
however,  germ  and  body  plasm  are  doubtless  one  and  the 
same  thing,  with  the  result  that  a  certain  amount  of 
inheritance  of  acquired  characters  can  and  does  take 
place.  Such  an  opinion  would  not  be  out  of  harmony 
with  the  views  of  Weismann,  who  was  early  forced  to  the 
belief  that  inheritance  of  acquired  characters  must  take 
place  in  the  more  primitive  organisms. 

The  opinion  of  the  biological  world  was  becoming 
fairly  well  settled  on  this  matter  when  Guyer's  startling 
results  (9)  were  published.  It  will  be  seen  that  Guyer's 
methods  ''strike  at  the  germ  plasm"  more  directly  than 
any  that  had  previously  been  tried. 

Grinding  up  the  eyes  of  white  rabbits,  Guyer  pro- 
cured a  lens-extract.  This  was  injected  into  the  blood 
stream  of  fowls.  There,  since  the  lens-extract  was  a 
foreign  and  "  inhamionious "  protein,  a  reaction  took 
place  which  resulted  in  the  production  in  the  blood 
stream  of  an  antibody  (following  the  same  principles 
which  apply  to  the  production  of  antitoxins  in  medicine). 
This  particular  antibody  had  the  peculiar  property  of 
"precipitating"  or  in  some  way  rendering  functionless 
the  characteristic  protein  of  rabbit  lens.  The  property 
is  quite  specific,  so  that  this  antibody  may  appropriately 


26  Outline  of  Genetics 

be  spoken  of  as  antilens.  Serum  obtained  from  the 
blood  of  fowls  thus  ''sensitized,"  and  therefore  contain- 
ing antilens,  was  injected  into  the  blood  stream  of  nor- 
mal white  rabbits.  No  noticeable  modification  was 
obtained  in  any  case  upon  adult  rabbits  that  were  so 
treated. 

When,  however,  the  serum  was  injected  into  pregnant 
mother-rabbits,  starthng  results  were  obtained.  Some 
of  the  resulting  progeny  had  eyes  that  were  clearly  defect- 
ive. Furthermore,  the  abnormality  was  readily  trans- 
mitted through  the  female  line  for  quite  a  number  of 
generations,  without  any  additional  injections  being 
made.  The  defect  did  not  decrease  in  degree,  but  seemed 
even  more  pronounced  in  the  later  generations. 

At  this  point  a  few  questions  might  be  asked.  Have 
such  eye  defects  ever  been  known  to  occur  among 
untreated  white  rabbits;  is  this  the  sort  of  thing  that 
might  appear  ''spontaneously"  through  mutation,  or  a 
recessive  character  that  might  have  been  segregated  out 
through  inbreeding,  as  is  true  of  so  many  other  functional 
abnormalities?  Careful  inquiry  has  revealed  the  fact 
that  no  such  eye  defects  have  been  reported  elsewhere. 

Again,  is  this  the  sort  of  thing  that  might  be  expected 
to  result  from  any  sort  of  mutilation,  or  is  it  a  specific 
response  to  a  specific  stimulus  ?  This  question  is  clearly 
answered  by  the  behavior  of  the  controls.  Untreated 
pregnant  mothers,  mothers  treated  with  serum  from 
unsensitized  fowls,  and  mothers  treated  with  serum  from 
fowls  that  had  been  sensitized  to  rabbit  tissues  other 
than  lens  never  gave  any  defective  progeny. 

An  even  more  critical  question  is  the  following:  is 
this  another  case  of  transmission  rather  than  true  inherit- 


The  Inheritance  of  Acquired  Characters  27 

ance;  whatever  may  be  the  material  basis  of  ilie  defect- 
ive eyes,  is  it  regularly  passed  from  tlie  body  of  the 
mother  to  the  young  in  utero  rather  than  through  the 
germ  cell  proper  ?  The  answer  to  this  question  was  early 
suggested  by  the  following  facts.  Later  litters  from  the 
mothers  that  had  originally  been  treated  never  contained 
any  defective  indi\dduals.  The  influence  of  the  anti- 
lens  seems  to  die  out  in  the  blood  stream,  suggesting 
that  it  is  only  by  being  incorporated  in  the  gemi  plasm 
that  the  character  can  be  perjoetuated.  A  more  con- 
vincing demonstration  of  this  point  appeared  in  the 
later  experiments.  Males  with  defective  eyes  when 
mated  with  females  from  a  normal  line  j:)r(3duced  only 
nonnal  offspring.  When,  however,  these  same  males 
were  remated  with  their  own  daughters  from  the  fore- 
going cross,  a  certain  number  of  defective  offspring 
resulted.  Evidently  the  abnormality  can  be  passed  on 
by  a  male  parent;  it  behaves  in  inheritance  like  a  Men- 
delian  recessive  character.  Inasmuch  as  the  male  parent 
exerts  upon  the  progeny  no  influence  beyond  what  is 
contained  in  a  single  male  gamete,  and  inasmuch  as  this 
gamete  is  practically  nothing  more  than  a  nucleus,  the 
foregoing  amounts  to  about  as  clear  a  demonstration  of 
inheritance  of  acquired  characters  as  would  be  possible. 
The  very  definite  results  of  this  set  of  ex[)eriments  are 
sufficient  to  prove  that  inheritance  of  acquired  char- 
acters can  take  place,  even  in  one  of  the  higher  animals 
where  germ  plasm  and  body  plasm  must  be  as  sharply 
differentiated  as  anywhere.  It  is  equally  true,  however, 
that  only  very  special  conditions  can  produce  the  result. 

A  word  might  be  said  011  the  theoretical  mechanism  involved. 
GuYER  proposes  that  "there  is  some  degree  of  constitutional 


28  Outline  of  Genetics 

identity,  probably  protein  homology,  between  the  mature  sub- 
stance of  a  tissue  and  its  correlative  in  the  germ,"  and  that  "basi- 
cally, inheritance  is  mainly  a  question  of  the  perpetuation  of  spe- 
cific protein-complexes,  and  development  the  result  of  differential 
reactions  of  these  same  fundamental  constituents  under  different 

conditions  of  environment Is  it  unreasonable  to  suppose  that 

if  changes  come  to  pass  which  affect  certain  constituents  of  tissue 
cells,  this  influence,  if  borne  in  the  circulating  fluids  of  the  body, 
could  also  affect  the  hom^ologous  constituents  of  the  germ  cells?" 

The  same  result  has  been  obtained  by  the  direct  injection  of 
lens-extract  into  the  blood  stream  of  the  rabbits  themselves.  It 
is  perhaps  surprising  that  rabbits  will  manufacture  antibodies 
for  one  of  their  own  tissues.  It  is  evident  that  they  do,  however, 
and  this  brings  us  a  distinct  step  closer  to  something  that  m'ght 
occur  under  natural  conditions.  If  degenerating  eyes  may  them- 
selves originate  antibodies  which  in  turn  affect  the  germ  cells,  the 
cardinal  principle  of  Lamarck's  theory  of  evolution  through 
inheritance  of  acquired  characters  must  be  conceded. 

The  fact  that  the  eyes  of  the  pregnant  mothers  were  in  no  case 
themselves  affected  does  not  necessarily  recommend  the  assumption 
of  "susceptibility"  only  in  the  embryonic  state,  but  is  doubtless 
due  merely  to  the  fact  that  there  is  a  relatively  much  greater  blood 
supply  to  the  lens  of  embryo  rabbits  than  those  of  adults. 

The  experiments  of  Griffith  and  Detlefsen,  final  results  of 
which  have  not  yet  been  published,  promise  to  provide  an  equally 
good  demonstration  of  inheritance  of  acquired  characters  in  mam- 
mals (see  Science  56:676-678.  1922). 

A  few  examples  of  the  supposed  inheritance  of  the 
''effects  of  environment"  in  plants  should  be  considered. 
Zedebaur  found  that  Capsella,  which  in  the  course  of 
many  years  had  gradually  crept  along  the  roadside  up 
into  an  alpine  habitat  and  there  acquired  alpine  char- 
acters, retained  these  characters  when  transplanted  to 
the  lowlands.  This  has  been  accepted  by  some  as  an 
authentic  instance  of  inheritance  of  acquired  characters; 
but  it  is  possible  that  this  conquest  of  an  alpine  habitat 


TJie  InJieritance  of  Acquired  Characters  29 

by  Capsella  can  better  be  explained  by  the  gradual  natu- 
ral selection  of  just  those  gemiinal  variations  that  best 
fitted  individuals  to  cope  with  alpine  conditions.  This 
would  result  in  the  gradual  establishment  of  a  strain 
of  germ  plasm  that  would  produce  body  structures  fitted 
to  alpine  conditions.  In  other  words,  this  is  just  the 
way  in  which  natural  selection  would  develop  a  new 
elementary  species  from  the  original  t}pe.  If  such  a 
type  were  established,  of  course  its  gcnn  phism  would 
produce  alpine  plants,  even  under  lowland  conditions. 
They  might  not  survive  long,  and  natural  selection  might 
eliminate  them,  but  their  structure  would  be  due,  not  to 
the  inheritance  of  somatic  structures,  but  to  the  inherit- 
ance of  an  alpine  germ  plasm. 

The  objection  to  Zedebaur's  conclusions  on  the 
grounds  that  the  result  may  be  attributed  to  natural 
selection  has  been  avoided  by  the  famous  experiments 
of  Bonnier  (5).  In  1884,  this  investigator  began  mak- 
ing plantations  in  the  lowlands  and  at  various  altitudes 
in  the  Alps,  so  arranged  that  the  two  individuals  to  be 
compared  were  produced  by  dividing  one  plant.  After 
a  lapse  of  over  thirty  years  he  has  made  the  following 
report.  A  few  of  the  plants  taken  from  the  plains  to 
alpine  stations  died,  but  a  list  is  given  of  fifty-eight  species 
that  proved  able  to  maintain  themselves  at  high  altitude. 
These  have  all  undergone  changes  which  make  them 
closely  resemble  indigenous  alpine  plants.'     In  at  least 

^  The  principal  chan<::cs  arc  relatively  lari^a-  (levelo|)ment  of  tiie 
subterranean  as  compared  with  aerial  parts,  shorteninj^  of  the  leaves  and 
of  the  internodes  of  stems,  increased  hairiness,  and  relatively  larger 
development  of  bark  and  protective  tissues.  The  leaves  become  thicker 
in  proportion  to  their  surface  and  are  a  deeper  green,  with  more  higlily 
developed  palisade  tissue  and  a  larger  numlx'r  of  ihloroplasts,  while 
the  flowers  are  larger  and  more  highl}'^  colored. 


30  Outline  of  Genetics 

seventeen  species  the  changes  are  so  great  that  the  plants 
have  apparently  been  transformed  into  distinct  alpine 
"species."  The  reverse  experiment,  transplanting  alpine 
plants  to  the  lowlands,  gave  similar  but  less  startling 
results. 

These  experiments  as  they  stand  are  really  more  serv- 
iceable to  the  ecologist  than  the  geneticist.  The  geneti- 
cist wishes  to  know  whether  the  transformations  will 
maintain  themselves  when  the  plants  are  returned  to 
their  original  stations  and  propagated  by  seed.  Bon- 
nier has  as  yet  made  no  clear  statement  on  this  latter 
point. 

An  interesting  issue  arises  in  this  connection.  If  the 
transformed  plants,  after  being  returned  to  their  original 
stations,  revert,  in  the  course  of  a  number  of  generations, 
are  we  to  conclude  that  inheritance  of  acquired  char- 
acters has  not  taken  place  ?  Should  we  not  rather  expect 
that,  if  inheritance  of  acquired  characters  takes  place 
under  a  given  set  of  conditions,  the  reverse  conditions 
will  bring  the  reverse  change  according  to  exactly  the 
same  principle  ?'  Such  work  as  that  of  Bonnier  may 
eventually  demonstrate  that  inheritance  of  acquired  char- 
acters is  a  possibiUty  in  plants,  though  it  may  fail  to 
demonstrate  that  irreversible  evolution  can  be  brought 
about  through  inheritance  of  acquired  characters.  The 
latter  can  be  fully  demonstrated  only  when  an  acquired 
character  comes  to  be  represented  by  a  gene  or  set  of 
genes  in  the  germ  plasm,  which  are  as  definitely  and 
"permanently"  a  part  of  the  hereditary  complex  as  any 

^  It  is  of  course  true  that  some  evolutionary  changes  are  probably 
irreversible  (Herrick  12),  but  such  changes  are  probably  not  involved 
in  the  Bonnier  experiments. 


TJie  Inlicrilance  of  Acquired  Characters  31 

of  the  pre-existing  genes.  Such  a  demonstration  has  as 
yet  not  been  approached  among  higher  ])lants;  it  is  aj)- 
proximated  for  animals  by  Guyer's  white  rabbit  experi- 
ments. 

Attention  should  be  called  to  another  ])hen()menon 
which  can  easily  be  confused  with  inheritance  of  acquired 
characters.  If  corn  is  planted  in  poor  soil,  weak  indi- 
viduals result.  Seed  from  these  weak  indi\nduals,  when 
planted  in  good  soil  will  develop  again  somewhat  weak- 
ened indi\iduals,  suggesting  the  inheritance  of  acquired 
characters.  This,  however,  is  merch'  the  direct  effect 
of  environment  continuing  through  the  second  genera- 
tion. The  weak  individuals  in  the  poor  soil  develop 
small  seeds  with  low  nutritive  capacity,  and  ])lants 
developed  from  abnormally  small  seeds  are  always  weak, 
whether  the  individual  that  produced  the  seed  grew  in 
poor  soil  or  not. 

There  has  been  fairly  good  agreement  on  the  point 
that  trees  deformed  by  prevailing  winds,  like  the  willows 
that  line  the  canals  in  Belgium  and  Holland,  or  storm- 
crippled  trees  along  exposed  seacoasts,  do  not  produce 
progeny  showing  these  characters  when  the  adverse 
environmental  conditions  are  removed.  Mavr  (15)  has 
written  a  notable  work  on  silviculture,  in  which  he  claims 
that  only  species  characters  are  inherited  in  trees,  and 
that  the  effects  of  climate  are  not  inherited,  and  there- 
fore that  the  source  of  the  seed  makes  no  dilTerence.  l\\ 
other  words,  seeds  of  Scotch  pine  would  alwa}s  produce 
Scotch  pine  progeny,  no  matter  at  Avhat  latitude  or  alti- 
tude the  ancestors  had  been  growing.  According  lo 
Mayr,  therefore,  there  is  no  inheritance  of  acquired 
characters  in  trees. 


32  Outline  of  Genetics 

Dr.  Arnold  Engler  (8)  found,  however,  that  in  the 
seedHngs  in  his  nursery  growth  in  height  distinctly 
decreased  as  the  altitude  or  latitude  from  which  the 
seed  came  increased.  He  also  found  that  seeds  from 
pines  which  had  been  crippled  by  growing  in  poor  soil 
conditions  gave  rise  to  crippled  plants  when  grown  in 
good  soil.  In  many  cases,  trees  of  the  third  generation 
still  showed  the  habit  "acquired"  by  their  grandparents 
in  different  habitats. 

These  are  striking  results,  but  it  is  well  to  bear  in 
mind  all  of  the  possibilities.  Engler  might  have  been 
'deahng  with  slightly  different  strains  of  trees,  differing 
in  germinal  constitution;  or  it  may  have  been  another 
case  of  the  "false  inheritance  of  acquired  characters" 
that  was  explained  in  connection  with  corn.  Seeds 
from  higher  latitudes  and  altitudes  might  well  have  been 
smaller,  so  that  we  should  have  expected  smaller  progeny, 
even  when  grown  in  the  lowlands.^ 

There  are  several  examples  of  what  seems  to  be 
inheritance  of  acquired  characters  in  simpler  plants,  but 
opinion  is  not  settled  on  interpretation  of  results.  Jen- 
nings' statement  (see  p.  24)  includes  the  bacteria. 
The  work  of  Hansen  (ii)  is  interesting.  This  investi- 
gator took  isolated  yeast  cells,  which,  when  cultivated 
under  ordinary  conditions,  uniformly  gave  rise  to  spore- 
bearing  forms,  and  subjected  them  for  a  time  to  the 
highest  temperature  at  which  growth  could  still  occur. 
As  a  result  he  procured  a  race  which  has  been  cultivated 
under  ordinary  conditions  for  twelve  years  without  once 
developing  spores. 

^  We  have  reason  to  believe  the  size  of  the  seed  ma3'  affect  the  size 
of  the  resulting  plant  even  in  such  forms  as  coniferous  trees  (Munns  i6). 


The  Inheritance  of  Acquired  Characters  33 

As  an  example  of  the  sort  of  thing  that  in:iy  occur  among 
fungi,  the  work  of  Long  (14)  may  be  mentioned.  Piicciuia  Eliis- 
iana  and  P.  Andropogonis  both  grow,  in  one  stage,  on  Aiidro- 
pogon,  where  they  are  to  be  distinguished  by  morphological  differ- 
ences in  the  uredospores.  P.  Ellisiana  has  Viola  for  its  alternate 
host,  while  P.  Andropogonis  has  Pcntcstemon.  P.  Ellisiana,  how- 
ever, has  been  artificially  induced  to  infect  Petitestcmon ,  where  it 
produces  spring  spores  that  resemble  those  of  P.  Andropogonis. 
When  these  spring  spores  are  returned  to  Andropogon,  the  result- 
ing uredospores  are  morphologically  identical  with  P.  Andropo- 
gonis uredospores.  Conversely,  P.  Andropogonis  can  be  made  to 
infect  Viola,  where  it  produces  morphologically  P.  Ellisiana  spring 
spores,  and  these  will  bring  P.  Ellisiana  uredospores  when  returned 
to  Andropogon. 

This  appears  to  be  an  actual  change  in  species  through  a 
change  in  the  quahty  of  the  nutrition.  But  can  it  be  demon- 
strated that  the  two  forms  were  really  distinct  "species"  in  the 
first  place? 

The  findings  made  by  Williams  (19)  on  periodicity 
of  sex  cells  in  the  marine  alga,  Dictyota  dichotoma,  rather 
clearly  indicate  inheritance  of  acquired  characters.  In 
any  one  locality  the  male  and  female  organs  originate 
simultaneously  and  pass  through  their  successive  stages 
of  development  together;  a  general  liberation  of  gametes 
and  fertilization  takes  place  on  a  particular  day.  This 
period  differs  between  different  localities  and  varies 
slightly  at  any  one  locality,  at  all  times  showing  a  clear 
relationship  to  the  tides,  and  therefore  to  the  amount  of 
light  that  reaches  these  submerged  plants.  So  intimate 
is  the  foregoing  relationship  that  one  can  })re(lict  the  time 
of  liberation  and  fertilization  by  consulting  the  almanac 
of  the  locality.  On  the  other  hand,  there  is  no  evidence 
of  periodicity  in  seas  where  there  are  no  tides.  Plants 
transferred    to    aquaria    in    the    laboratory,    and    thus 


34  Outline  of  Genetics 

removed  from  the  influence  of  tides  and  varying  light, 
continued  to  show  the  characteristic  periodicity  of  the 
locaHty  from  which  they  came.  Thus  an  obvious  adjust- 
ment on  the  part  of  the  plant  to  a  varying  set  of  environ- 
mental conditions  has  evidently  become  heredity. 

It  would  be  unwise  to  attempt  any  final  conclusions 
on  this  subject  of  inheritance  of  acquired  characters; 
the  status  of  the  subject  changes  as  new  evidence  is 
gathered.  Such  evidence  as  we  have  considered,  how- 
ever, recommends  the  following  tentative  conclusion. 
Inheritance  of  acquired  characters  is  possible  in  many 
organisms.  This  possibility  is  more  often  realized  per- 
haps in  the  simpler  than  in  the  more  complex  organisms. 
In  the  latter,  an  unusual  set  of  conditions  is  required, 
such  that  the  well-insulated  germ  plasm  will  be  reached. 

From  the  point  of  view  of  the  geneticist,  the  small 
amount  of  inheritance  of  acquired  characters  that  might 
take  place  is  usually  negligible.  The  geneticist  deals 
almost  entirely  with  characters  the  origin  of  which  is 
either  entirely  unknown  or  'Spontaneous"  (mutation) 
and  not  clearly  traceable  to  any  specific  environmental 
conditions.  For  the  evolutionist,  however,  this  phenom- 
enon becomes  very  significant.  The  recent  appearance 
of  seemingly  irrefutable  instances  of  inheritance  of 
acquired  characters,  taken  together  with  the  growing 
conviction  that  mutation  does  not  provide  the  type  of 
change  necessary  to  account  for  progressive  evolution 
(see  chapter  on  ' 'Mutation"),  is  leading  to  a  revival,  in 
modified  form,  of  the  Lamarckian  view. 


The  Inheritance  of  Acquired  Characters  35 

LITERATURE  CITED 

1.  Babcock,  E.  B.,  and  Claussen,  R.  E.,  Genetics  in  relation 
to  agriculture.     New  York:   McGraw  Hill  Book  Co.  1918. 

2.  Bateson,  W.,  The  progress  of  Mendelism.  Xature  104:214- 
216.  1919. 

3.  Blaringhem,  L.,  Production  par  traumatisme  d'unc  forme 
nouvelle  de  JMais  a  caryopses  multiples,  Zca  Mays  var.  poly- 
sperma.     Compt.  Rend.  Acad.  Sci.     Paris  170:677-679.  1920. 

4.  BoLLEY,  H.  L.,  The  importance  of  maintaining  a  constant 
elimination  factor  in  plant  breeding.  Ann.  Rep.  .Vmer. 
Breeders  Assoc.  8:508-514.  191 2. 

5.  Bonnier,  Gaston,  Nouvelles  observations  sur  les  cultures 
experimentals  a  diverses  altitudes  et  cultures  par  semis.  Rev. 
Gen.  Bot.  32:305-326.  pis.  2.  figs.  4.  1920. 

6.  Castle,  William  E.,  Genetics  and  eugenics.  Cambridge. 
1920. 

7.  DoNCASTER,  Leonard,  An  introduction  to  the  study  of  cytol- 
ogy.    Cambridge.  1920. 

8.  Engler,  Arnold,  Influence  of  source  of  seed.  Jour.  Heredity 
5:185-186.  1914. 

9.  Guyer,  M.  F.,  and  Smith,  E.  A.,  Studies  on  cytolysins.  II. 
Transmission  of  induced  eye  defects.  Jour.  Exp.  Zool.  31: 
171-223.  pis.  4.  figs.  7.  1920. 

10.  ,   Experiments   with   typhoid   agglutinins   in    rabbits. 

Anat.  Rec.  20:214.  1921. 

11.  Hansen,  Emil  Chr.,  Recherches  sur  la  physiologie  et  la  mor- 
phologic des  ferments  alcoliqucs.  XllL  Xouvellcs  etudes 
sur  des  levures  de  brasserie  a  fermentation  basse.  Compt. 
Rend.  Carlsberg  7:179-217.  figs.  S.  190S. 

12.  Herrick,  C.  J.,  Irreversible  differentiation  and  orthogenesis. 
Science  51:621-625.  1921. 

13.  Jennings,  H.  S.,  Variation  in  uniparental  reproduction. 
Amer.  Nat.  56:5-15  1Q22. 

14.  Long,  W.  H.,  Influences  of  the  host  on  the  moiphological 
characters  of  Puccinia  Ellisiatia  and  P.  Amlropogonis.  Jour. 
Agric.  Research  2:303-319.  1914. 


36  Outline  of  Genetics 

15.  Mayr,  H.,  Waldbau  auf  naturgesetzlicher  Grundlage.     Berlin. 
1909. 

16.  MuNNS,  E.  N.,  Effect  of  fertilization  on  the  seed  of  Jeffrey 
pine.     Plant  World  22 :  138-144.  1919. 

17.  PouLTON,  Shonland,  Shipley,  Weisaiann,  on  heredity.     Clar- 
endon Press.  1 89 1. 

18.  Walter,  H.  E.,  Genetics.     New  York.  1913. 

19.  WiLLL\MS,  J.  Lloyd,  Studies  in  Dictyotaceae  III:  Periodicity 
of  sex  cells  in  D.  dichotyma.    Ann.  Botany  19:531-560.  1904. 


CHAPTER  III 
MENDEL'S  LAW 

Mendel's  law  is  the  basis  of  all  work  in  genetics,  and 
should  be  understood  from  its  original  statement  to  its 
somewhat  complex  development.  In  1865,  Gregor 
Mendel  (3)  published  in  the  proceedings  of  a  local 
scientific  society  the  result  of  eight  years  of  breeding 
experiments.  The  publication  was  so  obscure  that 
scientific  men,  in  general,  did  not  see  it,  and,  in  addition 
to  this,  Darwinism  was  at  that  time  absorbing  the  atten- 
tion of  biologists.  For  these  two  reasons,  Mendel's 
work  remained  unnoticed,  and  of  course  unappreciated, 
until  it  was  discovered  in  1900  and  became  the  great 
classic  of  genetics.  Its  influence,  therefore,  dates  from 
1900  rather  than  from  the  year  of  its  publication. 

The  substance  of  Mendel's  experiments  is  as  follows. 
Wishing  to  discover  the  contributions  of  each  parent  to 
the  make-up  of  their  progeny,  he  chose  for  his  work  the 
simple  garden  pea,  which  would  breed  rai)idly,  and  ex- 
hibited well-marked  varieties.  To  magnify  his  results, 
he  secured  hybrids  by  crossing  distinctly  different  t\pes 
of  peas,  and  to  avoid  confusion  he  considered  on\y  one 
character  in  each  experiment.  For  example,  he  crossed 
peas  which  contrasted  in  character  of  lieight,  of  flower 
color,  and  of  seeds.  In  all  cases  he  obtained  similar 
results,  so  that  a  single  example  will  sulHce.  l^urther- 
more,  he  discovered  that  it  made  no  difference  whether 
the  staminate  parent   was  a  dwarf  and   the  pistillate 

37 


T 

X 
i 

I 

D 

}   I 

T            D 

> 

T               T     T     T 

D     T 

T      1      D              f 

Fig.    2. — Diagram    illustrating 

38  Outline  of  Genetics 

parent  tall,  or  vice  versa,  and  so  for  all  the  characters 

used.     In  other  words,  what  are  called  reciprocal  crosses 

gave  the  same  results. 

The  progeny  of  a  tall  parent  and  a  dwarf  parent  were 

all  tall.     This  generation  is  known  as  the  first  hybrid 

or  the  Fi  generation.     When  this  generation  was  inbred, 

the  progeny  was  made  up  of 

tall  and  dwarf  indi\dduals 

in   a   ratio  of   3:1.     This 

generation  is  known  as  the 

second  hybrid  generation  or 

the    F2    generation.     The 

dwarf    forms    of     the    F2 

generation    subsequently 

bred  true,  producing  only 
visible  results  of  Mendel's  experi-     ^^^^^^^      q^  ^^^  ^^^j  ^^ 
ments.     Cross  between  tall  parent 

(T)  and  dwarf  parent  (D)  gives    one-third    bred    true   and 

hybrid  progeny  which  are  all  tall;     tWO- thirds  split   up  in  juSt 

hybrid  progeny  inbred   gives   3:1     guch    a    3:1     ratio    as     did 
ratio  in  second  hybrid  generation;       ...  ,.  . 

inbreeding  each  of  these  four  indi-    their  unmediate  parents  of 

viduals  separately  gives  for  third     the  Fj  generation.      This  is 

hybrid  generation  results  indicated     expressed        diagrammati- 
in  bottom  line.  ,,       .       ^ 

cally  m  fig.  2. 

Mendel's  theoretical  explanation  of  this  behavior 
involved  three  distinct  theses. 

I.  Independent  unit  characters. — This  means 
that  an  organism,  although  representing  a  morphologi- 
cal and  physiological  unity,  from  the  standpoint  of  hered- 
ity is  a  complex  of  a  large  number  of  independent  herit- 
able units.  Thus  if  one  pea  plant  is  tall  and  another  one 
is  dwarf,  the  behavior  of  the  hybrid  produced  from  them 
with  reference  to  this  character  will  be  the  same,  no 


MendeW  Law  39 

matter  what  other  characters  the  i)arent-plants  may  have 
had.  In  other  words,  the  characters  are  independent 
units,  unaffected  by  other  characters  or  units.  The 
character  of  tallness  from  a  tall  plant  with  wrinkled 
seeds  or  purple  flowers  will  act  just  the  same  as  from  a 
tall  plant  with  smooth  seeds  or  white  flowers.  1\illness 
is  a  unit,  and  its  behavior  in  inheritance  is  independent 
of  all  other  units. 

2.  Dominance. — In  the  germ  plasm  there  are  certain 
determiners  of  unit  characters  which  dominate  during 
the  development  of  the  body,  causing  these  characters 
to  dominate  over  others  and  thus  become  visible.  The 
characters  dominated  over  and  thus  not  allowed  to 
express  themselves  are  called  recessive  characters.  These 
recessive  characters  may  be  present  in  the  germ  plasm, 
but  cannot  express  themselves  and  become  \'isible  as 
long  as  the  dominant  characters  are  present.  When  a 
dominant  character  is  absent,  however,  its  recessive 
alternate  is  free  to  express  itself  and  become  \'isible. 

For  example,  in  the  case  of  tall  and  dwarf  peas,  tall- 
ness is  a  dominant  character  and  dwarfness  is  its  alter- 
native recessive.  When  a  dwarf  appears,  therefore, 
there  is  present  no  dominant  tallness  to  suppress  it. 
In  the  Fi  generation  all  the  individuals  were  tall  because, 
although  they  had  all  received  the  recessive  character 
of  dwarfness  from  one  of  the  parents,  they  had  received 
the  dominant  character  of  tallness  from  the  other  i)arent, 
and  so  dwarfness  did  not  appear  in  an}-  of  them.  Such 
pairs  of  alternative  characters  are  now  commonly  called 
atlelomorphs.  Thus  tallness  and  dwarfness  are  allelo- 
morphs in  the  pea,  one  dominant  over  the  other,  which 
is  therefore  recessive. 


40  Outline  of  Genetics 

3.  Purity  of  gametes. — A  gamete  can  contain  only 
one  of  two  alternative  characters.  For  example,  it  may 
contain  the  character  for  tallness  or  for  dwarfness,  but 
not  both.  In  other  words,  allelomorphs  cannot  be  repre- 
sented in  the  same  gamete.  If  the  gamete  having  the 
character  for  tallness  unites  with  one  having  the  char- 
acter for  dwarfness,  the  resulting  zygote  will  have  both, 
but  will  produce  a  tall  individual  because  tallness  is 
dominant  to  dwarfness.  When  this  tall  hybrid  produces 
gametes,  however,  one-half  of  them  will  contain  the 
character  for  tallness  and  one-half  of  them  the  character 
for  dwarfness.  Thus  the  alternative  characters  are 
''segregated"  in  gamete  formation,  and  no  gamete  will 
have  both  characters. 

These  three  theses,  independent  unit  characters, 
dominance,  and  purity  of  gametes  (due  to  segregation), 
make  up  the  theoretical  explanation  of  Mendel's  law. 
Independent  unit  characters  was  of  course  a  necessary 
conception.  It  was  original  with  Mendel,  and  has  also 
been  original  with  other  investigators,  but  this  concep- 
tion does  not  represent  the  essential  feature  of  Mendel's 
law.  The  idea  of  dominance  had  been  somewhat  vaguely 
proposed  before  Mendel's  time.  In  the  old  literature 
on  animal  breeding  one  meets  theories  of  ''prepotency," 
which  were  proposed  again  and  again  before  the  dis- 
covery of  Mendel's  work  in  1900.  In  any  event,  Men- 
del was  the  first  to  formulate  definitely  the  theory  of 
dominance  among  unit  characters.  It  should  be  realized 
also  that  dominance  is  not  an  essential  feature  of  Men- 
del's theory.  Many  cases  are  known  in  which  domi- 
nance fails,  but  in  other  regards  the  Mendehan  inherit- 
ance is  strictly  followed. 


Mender s  Law  41 

The  essential  feature  of  Mi;ndi;i/s  theor}-  is  his  con- 
ception of  the  purity  of  gametes,  brought  about  by  the 
segregation  of  alternative  characters.  With  Mendel 
this  was  a  purely  theoretical  scheme,  but  since  his  time 
cytological  investigation  has  discovered  an  actual 
physical  mechanism  which  exactly  satisfies  the  require- 
ments of  Mendel's  scheme.  Every  li\ang  organism  is 
composed  of  cells,  and  these  cells  are  endowed  with 
nuclei.  Every  nucleus  contains  a  certain  number  of 
darkly  staining  bodies  known  as  chromosomes.  The 
number  of  chromosomes  is  always  tlie  same  for  a  gi\'en 
species.  At  the  cell  divisions  which  take  place  in  con- 
nection with  the  growth  of  the  body,  each  chromosome 
is  very  carefully  divided  in  half,  so  that  the  nucleus  of 
each  daughter-cell  has  exactly  the  same  equipment  of 
chromosomes  as  the  mother-nucleus.  The  exactness 
of  this  division  in  itself  suggests  that  the  chromosomes 
are  the  bearers  of  hereditary  characters,  since  none  of 
the  other  cell  constituents  seems  to  be  so  accurateh' 
divided  at  cell  division.  Even  more  significant  is  the 
behavior  of  the  chromosomes  in  connection  with  gamete 
formation.  At  that  time  it  becomes  evident  that  the 
chromosomes  exist  in  pairs;  thus  there  is  always  an 
even  number  of  chromosomes  in  every  bcxh'  cell  of  an 
organism.  The  two  components  of  each  pair  of  chromo- 
somes are  always  morphologically  identical.  When  tlie 
organism  forms  gametes,  a  cell  dixision  takes  place  which 
is  fundamentally  different  from  the  preceding  cell  divi- 
sions. At  this  division  no  splitting  of  the  individual 
chromosomes  takes  place;  instead,  the  chromosomes 
Hne  up  in  pairs  and  the  nature  of  the  division  is  such  as 
to  draw  apart  the  components  of  each  ])air.      This  is 


42  Outline  of  Genetics 

known  as  the  reduction  division,  for  each  of  the  resulting 
nuclei  has  the  reduced  number  of  chromosomes,  just 
half  of  the  characteristic  number  in  the  body  cells.  It  is 
important  to  remember  that  this  reduction  is  not  indis- 
criminate, but  always  involves  a  separation  of  the  two 
components  of  each  chromosome  pair.  It  is  the  reduc- 
tion division  that  gives  rise  to  the  gametes.  Gametes, 
therefore,  are  characterized  by  the  reduced  or  haploid 
number  of  chromosomes,  in  contrast  with  the  body  cells 
which  have  the  diploid  number.  Gametes  have  just  one 
representative  of  each  chromosome  pair  that  appears  in 
the  body  cells.  When  two  gametes  unite  at  fertihzation 
there  is,  of  course,  a  return  to  the  diploid  number  in  the 
resulting  zygote. 

This  is  exactly  the  mechanism  required  by  Mendel's 
scheme,  on  the  assumption  that  the  chromosomes  are 
the  bearers  of  hereditary  characters.  So  much  data  has 
accumulated  to  justify  this  assumption  that  it  will  be 
treated  as  an  established  fact  in  the  subsequent  descrip- 
tions. 

The  chromosome  mechanism  may  be  applied  to  the 
case  in  hand  as  follows.  For  convenience,  we  will  assume 
that  the  nuclei  of  the  body  cells  in  Mendel's  peas  have 
each  four  chromosomes  (two  pairs).  This  is  represented 
in  fig.  3.  In  the  case  of  a  tall  plant,  two  (one  pair) 
of  the  four  chromosomes  carry  the  character  for  tallness, 
that  is,  something  that  determines  the  production  of 
the  taU  character  in  the  somatoplasm.  This  unknown 
something  is  called  by  various  names  in  the  literature 
of  genetics;  for  the  present  we  shall  refer  to  it  as  a  deter- 
miner. In  our  illustration,  therefore,  two  of  the  four 
chromosomes  carry  the  determiner  for  tallness. 


Mendel's  Law 


43 


Fig.  3  shows  a  somatic  cell  with  the  diploid  niuiiber 
of  chromosomes.  Tii  the  fomiation  of  gametes,  this 
number  is  reduced  to  the  haploid  number,  which  is  in 
this  case  two.  The  diagram  shows  that  the  reduction 
division  separates  (segregates)  the  two  chromosomes 
carrying  the  determiner  for  tallness,  so  that  each  gamete 
contains  one.  This  occurs  for  the  other  characters  as 
well  as  for  that  of  tallness.     From  the  tall  plant,  therefore, 


© 

© 

© 
© 

Tall  Parent 

© 
© 

© 

© 

Dwarf  Parent 

Fig.  3. — Diagram  illustrating  behavior  of  chromosomes  in  Mexdel's 
cross  of  tall  and  dwarf  peas.  Large  rectangular  figures,  nuclei  of  zygotes 
or  mature  individuals;  large  circles,  gametes;  snij^l  circles  within  zygotes 
and  gametes,  chromosomes;  letters  on  chromosomes,  determiners  {T, 
tallness;  Z>,  dwarfness). 

all  the  gametes  will  contain  the  detenniner  for  t;\llness, 
and  from  a  dwarf  plant  all  of  the  gametes  will  contain 
the  determiner  for  dwarfness.  When  these  two  indi- 
viduals are  crossed,  the  zygote  will  contain  both  deter- 
miners, and  these  two  determiners  will  be  transmitted 
together  in  the  succeeding  cell  generations.  The  indi\id- 
ual  developed  from  such  a  zygote  will  of  course  be  tall, 
but  at  the  same  time  it  will  be  carr}ing  a  recessive  deter- 
miner for  dwarfness,  and  this  fact  will  be  shown  by  its 
behavior  in  breeding.     The  result  of  inbreeding  such  hy- 


44  Outline  of  Genetics 

brids  is  indicated  in  the  accompanying  diagram  (fig.  4), 
which  represents  the  chance  matings  of  the  two  kinds  of 
gametes.  The  obvious  results  are  three  tall  individuals 
and  one  dwarf.  This  is  the  so-called  monohybrid  ratio, 
which  means  the  ratio  when  a  single  pair  of  allelomorphs 
is  considered. 

Before  discussing  the  further  development  of  Mendel's 
law,  it  will  be  necessary^  to  explain  some  of  the  terminol- 
ogy of  genetics.  When  each  gamete  carries  the  same 
kind  of  determiner  the  resulting  zygote  is  said  to  receive 
a  double  dose;  when  a  zygote  receives  only  a  single  such 
determiner  it  is  said  to  receive  a  single  dose.  In  fig.  4, 
one  zygote  receives  a  double  dose  of  tallness  and  two 
others  a  single  dose.  These  phrases  are  more  or  less 
common  in  the  literature  of  the  subject,  but  the  more 
frequent  terminology  is  as  follows.  When  two  similar 
gametes  unite  to  form  a  zygote  it  is  called  a  homozygote; 
when  the  two  pairing  gametes  are  dift'erent  the  zygote  is 
called  a  heterozygote.  Using  this  terminology,  it  is  e\'ident 
that  the  3 :  i  ratio  of  the  F2  generation  is  really  a  1:2:1 
ratio,  as  follows:  i  homozygote  for  the  dominant  char- 
acter, 2  heterozygotes,  and  i  homozygote  for  the  recessive 
character.  The  1:2:1  ratio,  therefore,  is  the  significant 
one  and  appears  as  a  3 :  i  ratio  only  because  of  dominance. 

In  the  experiment  represented  in  fig.  4,  three  tall 
indi\dduals  appear  in  the  F2  generation.  Superficially  the 
individuals  look  alike,  but  it  is  realized  that  one  differs 
from  the  other  two  in  germinal  constitution,  for  one  will 
produce  only  one  kind  of  gamete,  while  the  other  two 
will  produce  two  other  kinds.  To  indicate  this  situation 
JOHANNSEN  (2)  has  introduced  some  appropriate  termin- 
ology.    Organisms  which  appear  to  be  alike,  regardless 


Menders  Law 


45 


of  their  germinal  constitution,  arc  said  to  be  phciiotypi- 
cally  alike,  or  to  belong  to  the  same  phoiolypc.  On  the 
other  hand,  organisms  having  identical  germinal  con- 
stitution are  said  to  be  genolypically  alike,  or  to  belong 
to  the  same  genotype.  From  the  standpoint  of  pheno- 
types  only,  Mendel's  F2  generation  shows  the  3:  i  ratio; 
but  if  genotypes  are  considered,  it  shows  the  1:2:1  ratio. 


®    ® 


Fig.  4. — Diagram  illustrating  behavior  ot  lirst  generation  (F,) 
when  inbred.  Illustrates  meaning  of  "segregation"  and  "purity  of 
gametes,"  and  how  chance  matings  of  Fi  gametes  result  in  3:  i  ratio  in 
F2  generation;  dwarf  individual  produced  only  by  zygote  in  lower  right- 
hand  corner. 


In  other  words,  this  group  of  foniis  contains  two  pheno- 
types  but  three  genotypes.  J- 

Referring  again  to  fig.  4,  several  things  ma>'  bet 
inferred.  It  can  be  seen  what  will  happen  in  the  F, 
generation  when  the  F^  individuals  arc  inbred.  The 
dominant  homozygote  will  ]:)roduce  onl}-  dominant 
homozygotes  in  the  F3  generation;  the  two  hctcrozygotcs 
will  split  up  in  the  F3  generation  in  the  same  1:2:1  ratio 
as  did  their  hybrid  parents  of  the  Fj  generation;     and 


46  Outline  of  Genetics 

the  recessive  homozygote  will  produce  only  recessive 
homozygotes. 

It  is  interesting  to  consider  what  would  happen  if  a 
heterozygote  were  crossed  with  a  homozygous  recessive. 
It  should  be  obvious  that  one  half  of  the  progeny  would 
be  pure  recessives,  while  the  other  half  would  be  hetero- 
zygotes,  that  is,  there  would  be  a  i :  i  ratio. 

Thus  far  we  have  considered  only  what  is  called  the 
monohybrid  ratio,  that  is,  the  ratio  obtained  from  one 
pair  of  contrasting  characters,  such  as  tallness  and  dwarf- 
ness.  The  next  step  is  to  consider  the  dihyhrid  ratio. 
Mendel  also  used  contrasting  flower  colors,  finding,  for 
example,  that  red  flower  is  dominant  to  white  flower. 
Introducing  this  pair  of  contrasting  characters  into  the 
situation  we  have  been  considering,  the  dihybrid  ratio 
will  be  the  result.  Crossing  a  tall,  rei-floweredjndivid- 
ual  with  a  dwarf,  white-flowered  individual,  it  is  evident 
that  all  the  Fi  or  first  hybrid  generation  will  be  tall,  red- 
flowered  individuals,  since  both  of  these  characters  are 
dominant.  In  the  F2  generation,  however,  the  following 
ratio  will  appear:  9  tall,  red;  3  tall,  white;  3  dwarf,  red; 
and  I  dwarf,  white.  This  9:3:3:1  is  the  dihybrid  ratio, 
the  explanation  of  which  is  indicated  in  fig.  5.  The 
question  may  be  raised  why  the  characters  for  tallness 
and  redness  are  not  represented  on  the  same  chromo- 
some. If  they  were,  the  result  would  be  a  simple  mono- 
hybrid  ratio,  except  that  the  tall  individuals  would 
always  be  red  flowered,  and  the  dwarfs  would  always  be 
white  flowered.  The  possibility  of  one  chromosome 
carrying  two  different  determiners  will  be  considered 
later,  but  at  present  we  shall  assume  that  these  deter- 
miners are  on  different  chromosomes. 


MendeVs  Law 


47 


Fig.  5  shows  that  we  are  deaUng  witli  two  homozy- 
gotes,  each  producing  only  one  kind  of  gamete,  so  that 
all  the  F,  progeny  are  similar,  both  phenot>pically  and 


© 
© 

© 
© 

TaU  Red  Parent 

© 
© 

©. 

© 

Dwarf  White  Parent 

© 
© 

© 
© 

Gametes 


©,© 
®© 


©3© 
®© 


D)  (T 
R)  (R 


®© 


©,© 


®  ® 


©,o© 
®® 

®  ® 


©3© 


©,© 

©  © 


D)     D 


R       R 


©:© 


©.© 

R^  (w) 


©,© 
®  ® 


©,© 
©"© 

®  ® 


Fig.  5. — Diagram  illustrating  ciihybrid  ratio.  Upper  part  shows 
how  original  parents  were  crossed  to  give  Fi  hyi)ri(l;  lower  i)art  shows 
Fi  hybrid  producing  four  kinds  of  gametes;  chance  matings  among  these 
gametes,  when  Fi  is  inbred,  results  as  indicated  in  the  large  set  of  squares 
and  explains  the  9:3:3:1  ratio  in  the  Fa  generation. 


48  Outline  of  Genetics 

genotypically,  that  is,  with  the  same  appearance  and  the 
same  germinal  constitution.  Each  of  these  Fi  indi\dd- 
uals  will  produce  four  kinds  of  gametes.  The  possible 
combinations  of  these  gametes  that  will  occur  when  the 
Fi  is  inbred  are  expressed  by  the  checkerboard.  The 
resulting  F2  involves  four  pheno types,  as  follows:  nos. 
I,  2,  3,  4,  5,  7,  9,  10,  13  are  tall,  red-flowered  individuals; 
6,  8,  14  are  tall,  white;  11,  12,  15  are  dwarf,  red;  16  is 
dwarf,  white.     This  acounts  for  the  9:3:3:1  ratio. 

It  will  be  noticed  that  nos.  1,6,  11,  and  16  are  homozy- 
gotes  and  therefore  will  breed  true;  but  the  rest  are 
heterozygotes,  either  for  one  pair  of  characters  or  for 
both,  and  these  will  split  into  various  types  upon  further 
breeding. 

The  higher  polyhybrid  ratios  run  into  quite  a  string 
of  terms,  but  involve  no  further  new  principles.  For 
example,  the  F2  phenot^-pic  ratio  for  the  trihybrid  is 
27:9:9:9:3:3:3:1,  invoKdng  nine  pheno types  (and  2 7 
genotypes),  but  it  can  easily  be  worked  out  by  the  same 
method  as  was  used  for  the  dihybrid. 

Thus  far  we  have  been  considering  Mendel's  law  in 
simple  form,  and  have  enlarged  but  little  upon  Mendel's 
original  statement.  The  value  of  the  law  is  apparent. 
Upon  its  republication  in  1900,  it  was  taken  up  by  biolo- 
gists, and  numerous  breeders  set  to  work  to  test  it.  As  a 
consequence,  data  for  and  against  it  began  to  accumulate. 
As  might  be  expected,  there  was  much  apparent  evidence 
against  the  law,  but  as  geneticists  developed  a  better 
conception  of  the  mechanism,  the  contradictory  evidence 
was  explained  away.  Almost  every  type  of  inheritance 
has  now  been  explained  according  to  Mendel's  law.  A 
few  of  the  more  important  cases  will  be  presentecj, 


Menders  Law  49 

PRESENCE  AND  ABSENCE  HYPOTHESIS 

Tliis  may  be  regarded  as  a  new  method  of  Mendelian 
thought.  It  was  first  suggested  by  Correxs  (i),  but 
later  was  worked  out  in  detail  by  other  geneticists.  It 
is  merely  a  different  way  of  regarding  the  MendeHan 
mechanism.  For  example,  in  the  case  of  a  hybrid 
obtained  by  crossing  tall  and  dwarf  parents,  the  result 
had  been  explained  by  Mendel  as  due  to  the  fact  that 
one  chromosome  bears  a  determiner  for  tallness  and  the 
other  one  of  the  pair  carries  the  detemiiner  for  dwarfness. 
In  other  words,  each  one  of  a  pair  of  allelomoq)lis  is 
represented  by  a  determiner,  two  detemiiners  thus  being 
present.  Dwarfness  in  this  case  would  be  the  result  of 
the  interaction  of  that  determiner  and  its  enviromuent 
during  the  development  of  the  body;  and  the  same  for 
tallness.  When  both  were  present,  howe\'er,  the  con- 
ception of  the  situation  w^as  as  follows.  The  determiner 
for  dwarfness,  setting  up  its  usual  series  of  reactions, 
early  became  paralyzed  by  the  determiner  for  tallness 
or  its  products.  This  result  was  called  the  dominance 
of  the  character  for  tallness.  It  was  as  if  the  detemiiner 
for  tallness  completely  prevented  the  acti\'ity  of  the 
determiner  for  dwarfness.  This  conception  was  apjxir- 
ently  borne  out  by  the  facts  and  was  the  ex]3lanation  of 
the  mechanism   generally  accepted. 

According  to  the  presence  and  absence  h\'])othesis, 
however,  the  situation  is  looked  at  from  a  different 
point  of  view.  Tallness  is  the  result  of  a  detemiiner, 
but  dwarfness  is  merely  the  result  of  the  absence  of  the 
detemiiner  for  tallness.  The  dominant  character  is 
produced  by  an  inheritable  detemiiner,  but  the  recessive 
character  appears  only  when  the  dominant  detemiiner 


50 


Outline  of  Genetics 


is  lacking.  This  conception  has  some  evident  advan- 
tages and  may  modify  the  previous  Mendelian  diagram, 
as  shown  in  fig.  6.  This  appears  to  be  a  simpler  mech- 
anism to  account  for  the  phenomenon  called  dominance. 
In  the  case  of  the  dwarf  form,  there  is  a  "normal"  course 
of  development;  in  the  case  of  the  tall  parent  or  hybrid, 
however,  an  additional  determiner  stimulates  cell  growth, 
or  cell  division,  or  both. 

This  hypothesis  introduces  some  additional  termin- 
ology suggested  by  Bateson.     In  our  illustration,  the 


Dwarf  Parent 


Gametes 


Fig.  6. — Diagram  showing  how  the  original  scheme  must  be  modi- 
fied to  satisfy  the  presence  and  absence  hypothesis. 

tall  parent  has  two  determiners  for  tallness  and  therefore 
Bateson  calls  it  duplex,  having  a  double  dose.  For 
the  same  reason,  the  Fi  individuals,  having  only  one 
determiner  for  tallness,  he  calls  simplex.  According 
to  the  same  terminology,  the  dwarf  parent  is  nulliplex 
with  respect  to  its  character  of  tallness. 

Additional  advantages  of  the  presence  and  absence 
hypothesis  will  appear  later  in  connection  with  a  con- 
sideration of  blending  inheritance  and  of  cumulative  fac- 
tors in  inheritance.  Attention,  however,  should  be  called 
to  the  fact   that  those  who  accept  the  presence  and 


MendeVs  Law 


SI 


absence  hypothesis  do  not  use  the  fomi  of  notation  thus 
far  used  in  explaining  Mendehan  inheritance.  Assume 
that  T  is  used  to  express  the  detenniner  for  tallness,  the 
same  letter  (/)  is  used  to  express  its  absence.  P^or 
example,  instead  of  using  D  for  dwarfness,  /  is  used  for 
''lack  of  tallness"  (fig.  7).  It  is  a  matter  of  conven- 
ience to  have  a  symbol  to  re])resent  the  recessive,  the 
absence  of  something  that  is  present  in  another  indi- 
vidual. 


Dwarf  Parent 


Cameti 


es 


■  Fig.  7. — Diagram  showing  how  presence  and  absence  scheme  is 
actually  used,  with  small  letter  representing  "absence." 


In  summary,  the  essential  difference  between  the 
presence  and  absence  hypothesis  and  that  of  dominant 
and  recessive  is  that  in  the  former  case  the  recessive 
determiner  has  no  existence  at  all,  while  in  the  latter 
case  it  exists,  but  is  in  a  latent  condition  when  associated 
with  the  dominant. 

As  a  matter  of  fact,  some  of  the  later  investigations  have 
revealed  cases  that  can  hardly  be  accounted  for  by  the  presence 
and  absence  hypothesis.  In  spite  of  these  recogni/.ed  e.\ceptions, 
however,  the  scheme  of  notation  employed  under  the  presence 
and  absence  hypothesis  has  proved  so  convenient  that  it  is 
almost  universally  employed. 


52  Outline  of  Genetics 

The  checkerboard  is  an  excellent  method  of  depicting  the 
mechanism  at  play,  but  the  same  results  can  be  obtained  much 
more  quickly  and  just  as  safely  by  another  method.  Dihybrid 
(and  other  polyhybrid)  ratios  can  be  obtained  by  multiplying 
together  the  ratios  of  the  monohybrid  components  (for  this  really 
amounts  to  the  same  thing  as  the  checkerboard).  In  the  present 
case,  one  monohybrid  component  gives  an  F2  ratio  of  3  tall:i 
dwarf,  while  the  other  monohybrid  component  gives  3  red :  i 
white.  (3  tall:  I  dwarf)  times  (3  red:i  white)  equals  9  tall,  red  13 
dwarf,  red  13  tall,  whiten  dwarf,  white. 

The  student  will  find  it  a  very  helpful  exercise  to  work  out 
for  himself  the  various  phenotypic  ratios  that  will  be  produced  by 
inbreeding  the  various  genotypes  shown  in  the  Fa  checkerboard, 
and  by  crossing  them  in  various  ways.  In  such  problems  the 
method  of  procedure  is  to  work  out  separately  the  results  for  the 
different  monohybrid  components  and  then  to  synthesize  these 
results.   The  synthesis  in  this  case  amounts  to  simple  multiplication. 

First  of  all,  one  must  familiarize  himself  with  the  various 
possible  ratios  encountered  in  connection  with  monohybrid  crosses. 
These  are  indicated  in  the  following  simple  table.  (In  every  case 
the  student  must  satisfy  himself  as  to  how  the  Mendelian  mechan- 
ism brings  about  these  results.) 

Phenotypic  ratio 
^^°^^  Red        White 

i?i?Xanything i      :     o 

Rr  XRr 3:1 

Rr  Xrr i     :     i 

rr    Xrr o     :      i 

The  solution  of  dihybrid  problems  then  becomes  a  simple 
matter.     For  example,  take  the  following  cases: 

TtRrXttRR. 
The  monohybrid  ratio  as  regards  height  is  i  tall :  i  dwarf,  while 
the  flower  color  ratio  is  i  red:o  white.     Multiplying  these  together 
gives  the  dihybrid  result,  i  tall,  red:  i  dwarf,  red. 

TtRrXTtrr. 
The  height  ratio  is  3  tall:i  dwarf,  while  the  color  ratio  is  i  red:i 
white,  which  results  in  the  dihybrid  ratio,  3  tall,  red:i  dwarf, 
red: 3  tall,  white:  i  dwarf,  white. 


Menders  Law  53 

BLENDS 

This  t^pe  of  inlieritance  when  first  discovered  was 
thought  to  be  in  direct  conihct  w-ith  Mendel's  law.  It 
is  a  case  in  which  dominance  seems  to  fail,  for  the  two 
alternative  characters  both  express  themselves  and  the 
result  is  an  average  between  them.  It  is  easy  to  explain 
this  situation  in  accordance  with  the  presence  and 
absence  h}'pothesis  without  any  violation  of  Men- 
del's law. 

The  classic  example  of  blending  inheritance  was 
presented  by  Correns  (i)  in  breeding  work  upon  Mira- 
hilis  Jalapa,  the  common  four-o'clock.  Correns  crossed 
red-flowered  and  white-flowered  varieties,  and  all  the 
hybrid  progeny  had  rose  pink  flowers.  This  was  a  color 
blend,  distinctly  intermediate  between  the  colors  of  the 
two  parents.  The  Fi  generation,  therefore,  seemed  to 
contradict  Mendel's  law  in  that  one  color  character  was 
not  completely  dominant  over  the  other.  The  real 
situation,  however,  appeared  in  the  F^  generation 
obtained  by  inbreeding  individuals  of  the  Fi  generation 
which  showed  the  blend.  By  inbreeding  the  pink 
hybrids  Correns  obtained  the  perfect  1:2:1  ratio,  that 
is,  I  red  like  one  grandparent,  2  pink  like  the  hybrid 
parent,  and  i  white  like  the  other  grandparent.  Segre- 
gation was  evidently  taking  place,  the  only  unusual 
thing  being  the  appearance  of  the  Fj  individuals,  and 
that  was  explained  immediately  as  failure  of  dominance 
(see  fig.  8). 

The  question  this  introduces,  therefore,  is  that  of  a 
mechanism  which  would  account  for  such  a  result.  The 
easiest  explanation  offered  is  that  the  red  parent  was  a 
homozygote  for  redness  (double  dose)  and  the  hybrid 


54 


Outline  of  Genetics 


a  heterozygote  (single  dose) ;  the  inference  is  that  a 
single  dose  produces  pink  while  a  double  dose  pro- 
duces red. 

A  theoretical  explanation  of  this  occasional  difference  in  the 
result  of  double  and  single  doses  is  as  follows.  Imagine  that  the 
body  cells  of  a  plant  have  a  certain  capacity  for  expressing  heredi- 
tary characters.  In  such  a  case,  just  as  a  given  quantity  of  sol- 
vent can  dissolve  only  a  given  amount  of  solute,  so  the  body  cells 


Red  Parent 


Gamete         Gamete 


White  Parent 


(R)p,nkQ 


^ggs 


wm"^ 


0P.® 


R)        (   r 
Pink 


r  )        (   r 
White 


Fig  8. — Diagram  illustrating  blending  inheritance,  discovered  by 
CoRRENS  in  Mirabilis  Jalapa. 


can  express  hereditary  characters  only  to  a  definite  limited  extent. 
In  the  four-o'clock  a  single  dose  of  redness  may  be  thought  of  as 
half-saturating  the  body  cells,  while  a  double  dose  completely 
saturates  them.  In  cases  showing  a  complete  dominance,  how- 
ever, a  single  dose  completely  saturates  the  cells  and  a  double  dose 
can  do  nothing  more.  This  analogy  assists  in  visualizing,  on  the 
one  hand,  the  necessary  mechanism  of  blends  (apparent  failure 
of  dominance),  and,  on  the  other  hand,  that  for  cases  of  complete 
dominance. 


MendeW  Law 


55 


Problems  dealing  with  determiners  for  which  dominance  is 
lacking  differ  from  those  where  dominance  is  present  only  in  so 
far  as  the  monohybrid  ratios  must  differ. 


Cross 


Dominance  present 
Red        White 


Phenotypic  ratio 

Dominance  lackinR 
(Here  phenotypic  and  gcno- 
typic  ratios  are  the  same) 

Red         Pink        White 


RRXRR I 

RRXRr I 

RRXrr i 

Rr  Xrr 3 

Rr  Xrr i 

rr    Xrr o 


o 
o 
o 
I 

I 
I 


I 
I 
o 
I 
o 
o 


o 
I 
I 

2 
I 

o 


o 
o 
o 
I 
I 
I. 


For  example,  take  the  following  case,  where  dominance  is  pres- 
ent for  the  height  character  and  absent  for  the  color  character: 

TtRrXttRr. 

The  monohybrid  ratio  as  regards  height  is  i  talhi  dwarf,  while 
the  color  ratio  is  i  red  12  pinkii  white.  Multiplying  these 
together  gives  the  dihybrid  result,  i  tall,  red:i  dwarf,  red:  2  tall, 
pink:  2  dwarf,  pink:  i  tall,  white:  i  dwarf,  w^hite. 

LITERATURE  CITED 

1.  CoRRENS,  C,  Die  neuen  Vererbungsgesetze.     Berlin.  191 2. 

2.  JoHANNSEN,    W.,    Elemente    der    exakten    Erblichkeitslehre. 

Jena.  1909. 

3.  Mendel,  G.,  Versuche  iibcr  Pflanzen-Hybriden.    \'erh.  Naturf. 

Vereins  in  Briinn.  4:1865. 


CHAPTER  IV 

THE  FACTOR  H\TOTHESIS 

Mendel  concluded  that  each  plant  character  depends 
upon  a  single  detenniner.  Inheritance,  however,  has 
proved  to  be  a  much  more  complex  phenomenon  than 
was  indicated  by  Mendel's  peas.  Ratios  have  appeared 
that  were  puzzling,  and  geneticists  have  been  forced 
to  the  conclusion  that  there  may  be  a  complex  of  deter- 
miners for  a  single  character.  This  conception  is  known 
as  the  factor  h\^pothesis,  and  much  of  the  growing  com- 
plexity of  genetics  has  developed  around  this  hypothesis. 
Previously  we  have  used  the  word  ''determiner,"  imply- 
ing Mendel's  idea  that  a  single  determiner  is  responsible 
for  the  development  of  a  plant  character,  and  this  has 
been  true  of  the  examples  of  inheritance  pre\iously 
considered.  It  is  understood,  now,  however,  that  a 
character  is  frequently  determined  by  the  interaction 
of  two  or  more  separately  heritable  factors,  and  hence 
the  factor  hypothesis.  The  distinction  between  factors 
and  determiners  should  be  clear.  In  case  only  one  heredi- 
tary unit  is  involved  in  the  production  of  a  character, 
this  unit  should  be  referred  to  as  a  determiner;  in  case 
two  or  more  units  interact  in  the  production  of  a  char- 
acter, these  3i,xe  factors .^ 

^  This  distinction  of  terms  has  pedagogical  value,  but  is  frequently 
violated  in  the  literature,  where  "factor"  is  frequently  used  in  the 
sense  of  "determiner."  A  less  restricted  term,  gene,  refers  to  the  heredi- 
tary unit  without  implying  whether  it  acts  as  a  factor  or  simple 
determiner. 

56 


The  Factor  Hypothesis  57 

I.  Complementary  factors. — This  is  the  simplest 
expression  of  the  factor  hypothesis;  it  may  be  illustrated 
by  some  of  East's  work  (3).  Crossing  red-grained  and 
white-grained  corn,  this  investigator  obtained  an  Fi 
progeny  which  was  all  red.  This  would  suggest  that  the 
F2  generation  would  show  3  red  to  i  white;  but  instead 
it  showed  9  reds  to  7  whites,  which  might  seem  to  violate 
the  Mendelian  method  of  inheritance.  It  is  quite  in 
accord  with  Mendel's  law,  however,  if  we  consider  that 
two  complementary  factors  are  necessary  to  produce 
the  red  character,  and  that  each  of  these  factors  is  inher- 
ited separately.  Such  a  situation  would  give  a  dihybrid 
ratio,  as  indicated  in  fig.  9.  It  will  be  seen  that,  out 
of  the  16  individuals  in  the  F2  checkerboard,  9  will  be 
red,  for  they  alone  contain  both  complementary  factors; 
the  other  7  will  be  white.  The  situation  is  thus  explained 
by  the  dihybrid  ratio;  but,  although  only  one  character 
is  involved,  that  character  depends  upon  two  comple- 
mentary factors. 

Another  situation  is  worth  noting.  No.  6  of  the 
checkerboard  is  white  because  it  contains  only  one  of 
the  necessary  factors;  no.  11  is  white  for  the  same  rea- 
son, but  its  germinal  constitution  is  just  the  opposite. 
What  would  happen  if  these  two  were  crossed  ?  There 
is  only  one  possibiUty,  since  each  is  a  homozygote  produ- 
cing only  one  kind  of  gamete.  The  result  would  be  red, 
and  thus  a  cross  between  two  whites  would  produce  only 
reds.  What  would  be  the  result  if  nos.  6  and  15  were 
crossed,  the  former  being  a  homozygote  and  the  latter  a 
heterozygote  ?  It  is  obvious  that  the  resulting  progeny 
would  be  one-half  red  and  one-half  white.  The  same 
result  would  be  secured  in  crossing  nos.  11  and  14.     A 


S8 


Outline  of  Genetics 


cross  between  nos.  14  and  15,  both  of  which  are  het- 
erozygotes,   would   produce  3  white   to   i   red.     These 


Fig.  g. — Diagram  illustrating  behavior  of  complementary  factors 
in  cross  between  red-grained  and  white-grained  corn.  R  and  C  must 
both  be  present  to  produce  red-grained 


corn. 


illustrations  show  how  differently  the  same  phenotype 
may  behave  in  inheritance.  In  each  case  2  whites  were 
crossed,  that  is,  the  same  phenotypes,  but  3  different 


The  Factor  Hypothesis  59 

ratios  were  obtained  because  the  genotypes   were  dif- 
ferent. 

The  striking  feature  of  this  situation  is  that  one  can 
cross  two  whites  and  get  a  red.  This  gives  an  insight 
into  the  so-called  phenomenon  of  reversion.  For  ex- 
ample, in  the  course  of  numerous  breeding  experiments 
Bateson  (i)  obtained  two  strains  of  white  sweet  peas, 
each  of  which  when  normally  *'selfed"  bred  true  to  the 
white  color;  but  when  these  two  were  artilicialh'  crossed 
all  of  the  Fi  progeny  had  purple  flowers,  like  the  wild 
Sicilian  ancestors  of  all  cultivated  varieties  of  the  sweet 
pea.  This  appeared  to  be  a  typical  case  of  reversion. 
Further  breeding,  however,  showed  that  this  was  just 
such  a  case  of  complementary  factors  as  we  have  been 
considering.  One  of  Bateson's  white  strains  had  one 
of  the  factors  for  purple  flower  color  and  the  other  strain 
had  the  other  factor. 

It  is  interesting  to  note  that  if  an  investigator  should  cross 
homozygote  no.  i  with  homozygote  no.  11,  the  Fx  and  F2  results 
would  lead  him  to  conclude  that  the  red  character  was  due  to  a 
simple  Mendelian  determiner.  R  would  remain  a  ''determiner" 
until  a  strain  of  corn  was  discovered  which  lacked  the  C  factor; 
crosses  with  such  a  strain  would  reveal  the  real  mechanism  of  the 
situation,  and  thereafter  R  would  be  known  as  a  "factor." 

Complementary  factors  have  been  defined  and  the  method  of 
their  inheritance  described,  but  is  there  any  mechanism  to  explain 
the  situation  ?  A  suggestion  may  be  obtained  from  plant  chemis- 
try (2).  The  most  prominent  group  of  pigments  in  plants  is  the 
group  of  anthocyanins,  which  are  produced  as  follows.  Plants 
contain  compounds  called  chromogens,  which  are  colorless  them- 
selves, but  which  produce  pigment  when  acted  upon  by  certain 
oxidizing  enzymes  or  oxidases.  This  would  provide  a  mechanism 
to  account  for  the  behavior  of  complementary  factors.  If  one  of 
East's  white  strains  of  corn  contained  a  chromogcn  capable  of 


6o  Outline  of  Genetics 

producing  red,  but  lacked  the  necessary  oxidase,  it  would  remain 
colorless.  If  the  other  white  strain  contained  the  oxidase  but  no 
chromogen,  it  too  would  be  colorless.  In  crossing  them,  however, 
chromogen  and  oxidase  would  be  brought  together  and  a  red- 
grained  hybrid  would  be  the  result.  In  breeding,  such  red-grained 
individuals  would  of  course  give  red  and  white  progeny  in  a  ratio 
of  9:7,  as  explained  in  connection  with  East's  corn. 

It  should  be  realized  that  the  foregoing  is  no  more  than  a 
suggestion.  So  far  as  the  genetics  of  the  situation  is  concerned, 
complementary  factors  may  be  regarded  as  an  established  fact; 
but  what  either  one  of  these  factors  actually  amounts  to,  in 
physiological  terms,  has  as  yet  only  been  guessed  at.  In  fact,  it 
would  be  safe  to  state  that  there  is  no  known  case  where  the  exact 
physico-chemical  nature  of  any  factor  or  determiner  has  been 
demonstrated.  In  the  foregoing  instance,  neither  chromogen 
nor  oxidase  may  be  the  effective  units  involved.  There  are,  how- 
ever, plenty  of  possibilities  within  the  field  of  chemistry  where 
the  bringing  together  of  two  inert  substances  initiates  a  reaction 
sufficient  to  result  in  a  new  plant  "character."  Within  limits, 
hybridizing  is  very  much  like  mixing  chemicals  in  a  test  tube. 

The  origin  of  complementary  factors  is  an  interesting  field 
of  speculation.  Did  they  originate  together  or  separately?  A 
natural  inference  would  be  that  they  originated  together,  for 
neither  would  be  of  any  use  without  the  other.  It  should  be 
remembered,  however,  that  the  Darwinian  idea  of  usefulness  as 
explaining  the  occurrence  of  everything  in  a  plant  is  frequently 
inadequate.  One  must  think  rather  of  a  plant  as  a  complex  physi- 
co-chemical laboratory.  No  one  claims  that  all  chemical  reactions 
are  useful;  they  are  simply  inevitable;  and  plant  characters  are 
the  result  of  chemical  reactions  and  physical  necessities. 

The  other  alternative  is  to  suppose  that  these  factors  origi- 
nated independently  in  the  plant's  history.  In  this  case,  of 
course,  the  first  to  be  produced  would  remain  functionless  until 
its  complement  came  into  existence.  This  might  be  an  explana- 
tion of  what  have  been  called  "latent"  characters.  Also,  not 
only  might  they  have  originated  at  different  times  but  in  different 
varieties  or  species.    In  this  case,  if  natural  hybridizing  should 


The  Factor  Hypothesis  6i 

bring  them  together  the  result  would  be  the  appearance  of  a  new 
character;  a  few  authors  (notably  Lotsy  5)  believe  that  this  is  a 
very  important  factor  in  the  origin  of  species. 

The  behavior  of  the  red-grained  and  white-grained 
corn  serves  as  an  introduction  to  the  factor  hypothesis 
and  as  an  illustration  of  one  of  the  important  t}'pes  of 
factor  interactions.  Complementary  factors  are  always 
to  be  recognized  by  the  following  behavior.  A  comple- 
mentary factor  interacts  with  a  dissimilar  factor  to 
produce  a  particular  character. 

2.  Supplementary  factors. — A  supplementary  fac- 
tor interacts  with  a  determiner  (or  factor  complex)  to 
modify  the  character  produced  by  the  latter.  An  excel- 
lent illustration  of  this  factor  type  can  also  be  provided 
by  the  inheritance  of  color  in  grains  of  corn  (East  3). 
Interacting  with  the  factors  R  and  C  is  a  third  factor  P} 
This  P  factor  is  inherited  quite  independently  of  the 
other  two,  but  whenever  it  is  present  the  red  color 
becomes  modified  to  purple.  The  behavior  of  this  factor 
is  revealed  when  we  consider  the  phenotypes  to  which 
the  following  homozygotes  belong.  Corn  of  the  formula 
PPRRCC  has  purple  grains,  ppRRCC  is  red  grained, 
while  PPRRcc  and  PPrrCC  are  both  white  grained. 
From  these  facts,  one  can  draw  the  following  conclu- 
sions: (i)  when  P,  R,  and  C  are  all  present  a  pur])le 
grain  results;  (2)  red  color  can  appear  only  when  P  is 
absent;  (3)  P  itself  is  entirely  ineffective  in  the  absence 
of  either  R  or  C. 

*  In  the  literature  on  inheritance  in  corn,  this  factor  is  referred  to 
by  the  symbol  Pr,  its  absence  being  denoted  b>'  pr  (see  work  of  Emerson 
and  others).  It  is  to  avoid  possible  confusion  in  the  mind  of  the  student 
that  it  is  referred  to  by  the  single  letter  in  the  present  text. 


62  Outline  of  Genetics 

These  principles  reveal  the  nature  of  supplementary 
factors.  Unlike  complementary  factors,  they  never 
produce  characters,  but  merely  modify  characters  already 
present.  Otherwise  they  follow  the  same  principles  of 
independent  Mendelian  inheritance  with  which  we  are 
now  familiar. 

The  student  will  find  a  very  useful  exercise  in  the  solution  of 
various  problems  which  involve  simultaneously  several  types  of 
factors.  In  solving  such  problems,  it  is  neither  necessary  nor 
advisable  to  employ  any  "rule-of-thumb"  method.  As  usual 
the  checkerboard  can  be  depended  on  to  provide  an  accurate  solu- 
tion, but  this  is  too  slow  and  cumbersome.  Instead,  the  problems 
can  always  be  solved  rather  easily  by  keeping  in  mind,  at  all  times, 
the  fundamental  mechanism  of  inheritance  that  is  at  play  and 
proceeding  with  the  solution  in  a  logical,  orderly  manner.  (If 
the  student  simply  remembers  the  Mendelian  mechanism  he  can 
really  work  out  his  own  methods  of  solving  the  problems.) 

In  attacking  polyhybrid  problems,  the  first  principle  to 
remember  is  to  work  out  separately  the  solution  for  each  pair  of 
allelomorphs,  and  then  to  put  together  these  monohybrid  solu- 
tions. This  ''putting  together,"  in  the  case  of  simple  determiners, 
amounts  merely  to  multiplication,  as  was  described  before,  but 
where  factors  are  at  play  other  methods  of  putting  together  are 
necessary.  In  every  case,  the  method  of  putting  together  the 
monohybrid  solutions  is  a  rather  obvious  one,  and  is  clearly  indi- 
cated by  the  definition  of  the  factor  type  with  which  one  is  dealing. 

A  few  examples  involving  the  P,  R,  and  C  factors  will  serve  to 
illustrate.  In  these  cases,  one  must  consider  the  P  factor  last, 
since  it  is  effective  only  when  both  R  and  C  are  present. 

PPRyccXPpRRcc. 

Considering  first  the  C  factor,  one  finds  that  a  o:  i  ratio  will  result; 
that  is,  all  of  the  progeny  will  lack  the  C  factor.  Since  this  is  the 
case,  one  need  go  no  farther  with  the  solution,  since  absence  of  C 
is  sufiicient  in  itself  to  insure  that  all  of  the  progeny  will  be  color- 
less, or  white. 

PpRrCcXppRrCC. 


The  Factor  Hypothesis  63 

Considering  first  the  C  factor,  one  finds  that  a  i  :o  ratio  will  result; 
that  is,  all  of  the  progeny  will  have  the  C  factor.  So  far  as  C  is 
concerned,  then,  all  of  the  progeny  may  be  colored. 

Taking  up  now  the  R  factor,  it  is  evident  that  this  will  bring 
a  3 : 1  ratio;  that  is,  f  of  the  progeny  have  both  C  and  R  and  will 
therefore  be  colored  (whether  red  or  purple  will  be  decided  later) ; 
I  have  C  but  lack  R  and  will  therefore  be  colorless.  The  final 
fate  of  this  I  is  thus  decided,  and  one  need  give  no  further  atten- 
tion to  this  group.  The  f ,  however,  is  eligible  to  be  carried  on 
for  further  consideration  under  the  P  factor. 

The  ratio  produced  by  the  P  factor  is  1:1;  that  is,  ^  have  P 
and  I  lack  it.  So  far  as  phenotypes  are  concerned,  however,  the 
present  fractionation  applies  only  to  the  f  that  was  carried  over, 
making  |  with  and  |  without  P. 

Summarizing,  }  have  C  but  lack  R  and  are  therefore  colorless; 
I  have  C  and  R  but  lack  P  and  are  therefore  red;  |  have  C,  R,  and 
P  and  are  purple.  The  resulting  phenotypic  ratio  is,  therefore, 
3  purple: 3  red:  2  white. 

PpRrCc  ''selfed"  (equivalent  to  PpRrCcXPpRrCc). 

Considering  the  C  factor,  the  result  is  a  3 :  i  ratio,  or  |  with  and 
\  without  C.  The  latter  group,  \  is  hereby  dropped,  since  it 
must  be  colorless,  while  the  f  is  carried  on  for  further  con- 
sideration. 

The  R  factor  also  gives  a  3 :  i  ratio,  or  f  with  and  \  without 
R.  But  this  fractionation  applies  only  to  the  f  that  was  carried 
on.  Therefore  we  have  f  Xf  or  A  which  has  both  C  and  R,  and 
f  Xj  or  -i\-  which  has  C  but  lacks  R.  This  x\  is  hereby  dropped, 
since  it  also  must  be  colorless,  while  the  1^0  is  carried  on  for  further 
consideration. 

The  P  factor  gives  3:1,  or  f  with  and  \  without  P.  This 
fractionation  applies  to  the  iV  which  was  carried  on,  so  that  we 
have  iVX  4  or  H  which  has  C,  R,  and  P  and  is  purple,  and  ^i  Xi 
or  i;^  w^hich  has  C  and  R,  but  lacks  P,  and  is  therefore  red. 

Summarizing,  \  plus  i'',;  or  (jv  is  colorless;  ,m  is  red;  and  l\ 
is  purple.  The  phenotypic  ratio,  then,  is  27  purple:9  red:  28 
white. 


64  Outline  of  Genetics 

(Any  such  mass  of  fractions  is  readily  handled  on  the  black- 
board or  scratch  pad  by  considering  the  C,  R,  and  P  ratios  under 
separate  columns,  and  scratching  out  fractions  as  they  are  dis- 
posed of  by  being  carried  on.  At  the  end,  if  the  procedure  has 
been  orderly,  the  proper  fractions  will  be  found  under  the  proper 
columns.) 

Other  methods  of  solving  these  problems  could  be  worked  out. 
All  that  is  necessary  in  any  method  is  clear  vision  of  the  Mendelian 
mechanism,  analysis  into  monohybrid  components  and  separate 
solution  of  each,  followed  by  an  orderly  synthesis,  the  method  of 
synthesis  always  being  indicated  by  the  nature  of  the  factors  with 
which  one  deals. 

As  before,  the  physiological  mechanism  accounting  for  the 
behavior  of  the  P  factor  has  been  only  guessed  at.  It  has  been 
found  that  the  purple  pigment  is  produced  by  the  same  substance 
as  the  red,  but  represents  a  higher  state  of  oxidation,  which  sug- 
gests the  following  possibility.  C  is  oxidized  by  7?  up  to  a  certain 
point,  where  red  is  produced;  P,  an  additional  enzyme,  is  capable 
of  oxidizing  the  red  pigment  still  further  to  purple.  P  is  incapable 
of  attacking  the  original  chromogen,  but  when  R  carries  the 
attack  to  a  certain  point,  P  can  function  and  carry  the  oxidization 
further.  As  a  consequence,  P  without  R  gives  white  grains,  while 
R  gives  red  grains  only  in  the  absence  of  P. 

3.  Inhibitory  factors. — ^An  inhibitory  factor  pre- 
vents the  action  of  some  other  determiner  or  factor. 
This  factor  type  also  can  be  illustrated  in  connection 
with  inheritance  of  grain  color  in  corn  (East  3).  If  one 
were  to  cross  a  purple-grained  race  of  corn,  having  the 
formula  PFRRCC,  with  any  white-grained  race,  he  would 
expect  all  of  the  Fi  generation  to  be  purple.  In  this 
case,  every  gamete  produced  by  the  purple  parent  would 
have  the  formula  PRC.  As  a  consequence,  no  matter 
what  might  be  the  formula  of  the  white  parent,  every 
Fi  individual  would  have  to  have  at  least  one  dose  of 
P,  R,  and  C,  and  this  in  itself  should  be  sufficient  to 


The  Factor  Hypothesis  65 

insure  the  production  of  the  purple  color.  As  a  matter 
of  fact,  just  this  result  has  been  actually  realized  in 
practically  all  such  crosses.  In  one  case,  however, 
startlingly  different  results  were  obtained.  Crossing 
just  such  a  purple  parent  as  was  mentioned  with  a  white- 
grained  race  resulted  in  an  Fi  generation  which  was  all 
white. 

Inbreeding  this  Fi  gave  an  F2  generation  which  con- 
tained some  colored  grains,  but  a  decided  majority  of 
whites.  Evidently  the  colorless  condition  is  dominating 
over  the  colored.  This  would  be  surj^rising  in  any  case 
of  color  inheritance,  for  we  always  expect  colored  to  be 
due  to  the  presence  of  something  that  is  absent  in  col- 
orless. 

Analysis  of  the  present  case  revealed  the  fact  that 
the  white-grained  race  that  had  been  used  was  homozy- 
gous for  the  presence  of  an  inhibitory  factor,  /;  whenever 
this  factor  is  present  no  color  of  any  kind  can  be  produced. 
This  readily  explains  the  foregoing  results.  If  the  purple- 
grained  race  that  was  used  had  the  fonnula  iiPPRRCC 
and  the  white-grained  race  was  IIPPRRCC,  the  result- 
ing Fi  would  be  IiPPRRCC,  which  would  be  pheno- 
t}^pically  white  owing  to  the  presence  of  the  /  factor. 
Inbreeding  would  then  result  in  the  following  F2:  i 
IIPPRRCC,  phenot>TDically  white;  2  IiPPRRCC,  white; 
I  iiPPRRCC,  purple.  It  is  clear  that  the  colorless  con- 
dition is  actually  dominating,  but  it  is  dominating  on 
account  of  the  presence  of  the  /  factor. 

It  is  evident  that  the  purple  and  red  types  with  which 
we  had  been  dealing  before  must  all  have  been  homozy- 
gous for  the  absence  of  /.  Corn  grains  can  be  colorless 
for  any  of  three  reasons,  absence  of  R,  absence  of  C,  or 


66  Outline  of  Genetics 

presence  of  /;  whereas  the  colored  condition  occurs  only 
when  three  conditions  are  simultaneously  satisfied, 
presence  of  R,  presence  of  C,  absence  of  /. 

Tetrahybrid  problems,  involving  /,  P,  R,  and  C  can  be 
worked  out  on  the  same  principles  as  were  previously  outlined. 
It  is  probably  most  convenient  to  consider  the  /  factor  first. 

UPpRrCc  X  iippRRCc. 

Considering  the  /  factor,  there  will  be  |  of  the  progeny  with  and 
^  without  /.  This  first  |  is  now  dropped  because  it  is  bound  to 
be  colorless  on  account  of  the  presence  of  /,  no  matter  what  may 
be  the  rest  of  the  germinal  composition;  and  in  this  case  it  is  the 
latter  ^  that  is  eligible  to  be  carried  on  for  further  consideration. 

The  C  factor  gives  f  with  and  \  without  C.  Applied  to  the 
\  that  was  carried  on,  this  becomes  ^Xf  or  f  without  /  and  with 
C,  and  |Xi  or  |  without  /  and  without  C.  This  last  \  is  hereby 
dropped,  since  it  lacks  C  and  must  be  colorless;  while  the  f  is 
carried  on  for  further  consideration. 

The  R  factor  gives  a  ratio  of  i  with:o  without  R.  Conse- 
quently no  further  fraction  is  dropped  into  the  white  phenotype 
at  this  point,  and  the  whole  f  is  carried  on. 

The  P  factor  gives  \  with  and  \  without  P.  Applied  to  the 
I,  this  becomes  f  X|  or  A  which  lacks  /  and  has  C,  R,  and  P, 
and  is  therefore  purple;  and  f  Xf  which  lacks  /,  has  C  and  R,  and 
lacks  P,  and  is  therefore  red. 

Summarizing,  |  plus  |  or  if  is  colorless,  A  is  red,  and  A  is 
purple.     The  phenotypic  ratio  is,  3  purple  13  red:  10  white. 

liPpRrCc  ''selfed." 

/  gives  f  with  I:\  without  /.     Drop  the  f  and  carry  on  the  \. 

C  gives  f  with  C:\  without  C.  Applied  to  the  \  that  was 
carried  on,  this  becomes  |Xf  or  A  which  lacks  /  and  has  C;  and 
|Xi  or  j^,i  which  lacks  /  and  C  both.  Drop  the  Vo  and  carry  on 
the  A. 

R  gives  f  with  R :  \  without  R.  Applied  to  the  iV  that  was 
carried  on,  this  becomes  i^Xf  or  if  which  lacks  /  and  has  both 
C  and  R;  and  AXi  or  ^^  which  lacks  /,  has  C,  and  lacks  R. 
Drop  the  6^4  and  carry  on  the  o^. 


The  Factor  Hypothesis  67 

P  gives  f  with  P\\  without  P.  AppUed  to  the  A  that  was 
carried  on,  this  becomes  o^  Xf  or  /5V  which  lacks  /  and  has  C,  R, 
and  P,  and  is  therefore  purple;  and  ^aX\  or  of.n  which  lacks  /, 
has  C  and  R,  and  lacks  P,  and  is  therefore  red. 

Summarizing,  f  plus  iV  plus  ^t^  or  .7^,1  is  colorless,  250  is  red, 
and  2^5^  is  purple.  The  phenotypic  ratio  is,  27  purple  19  red:  220 
white. 

Thus  three  factor  types  are  at  play  in  the  inheritance  of  color 
in  grains  of  corn.  Four  distinct  factors  are  interacting,  but  all 
are  inherited  independently  and  quite  in  accordance  with  Men- 
delian  principles. 

Again,  the  exact  physiological  nature  of  the  /  factor  is  not 
understood,  but  can  be  only  guessed  at.  We  have  assumed  that 
color  is  produced  when  an  enzyme  is  present  to  oxidize  a  chromo- 
gen.  Enzymes  are  sensitive;  their  activities  may  be  affected  or 
completely  checked  by  various  agents.  Assume  that  /  is  such 
an  agent,  and  the  necessary  mechanism  is  provided.  When  /  is 
present  R  is  paralyzed,  so  that  it  cannot  oxidize  C. 

4.  Cumulative  factors. — These  are  considerably 
different  from  the  other  types  and  will  be  considered  in 
another  chapter  under  the  caption  "Inheritance  of 
quantitative  characters." 

These  four  great  factor  types  are  really  the  only  ones 
encountered  in  genetics,  each  representing  a  distinct 
type  of  interaction.  In  the  literature  of  the  subject 
many  other  descriptive  titles  are  given  to  factors,  but 
no  fundamentally  new  mechanisms  are  introduced. 

A  few  more  words  might  be  said  on  the  three  factor  types  that 
have  already  been  considered.  Of  these  three,  the  rarest  type  is 
the  inhibitory  factor,  complementary  and  supplementary  factors 
being  quite  common. 

It  is  not  surprising  to  lind  that  true  doniiiKint  inhibitory 
factors  are  rather  rare,  as  is  suggested  by  the  following  reasoning. 
In  nature,  there  has  been  for  countless  generations  a  struggle  for 
existence  among  the  individuals  of  a  species,  with  a  survival  of  the 


68  Outline  of  Genetics 

fittest.  It  follows  from  this  that  there  has  been  a  struggle  for 
existence  among  unit  characters.  Those  dominant  unit  char- 
acters which  are  "fit,"  which  serve  better  to  adapt  the  organism 
to  environmental  conditions,  are  the  ones  to  survive;  while 
"unfit"  dominant  unit  characters  are  eliminated  with  the  elimi- 
nation of  the  organisms  that  contain  them.  (The  same  reasoning 
would  not  apply  to  the  recessives,  which  are  "protected"  from 
natural  selection  when  they  occur  in  heterozygotes.)  These 
dominant  unit  characters  which  exist  today  are,  for  the  most  part, 
"survival"  characters,  being  important  in  the  economy  of  the 
organism  since  they  are  serving  to  adapt  it  to  the  environment. 
In  addition,  there  must  of  course  be  a  number  of  "indifferent" 
characters,  which  cannot  be  construed  as  adaptations;  these 
have  persisted  simply  because,  since  they  have  neither  positive 
nor  negative  survival  value,  there  has  been  no  reason  to  eliminate 
them.  Since  most  of  the  dominant  unit  characters  have  positive 
survival  value,  it  follows  that  anything  which  prevents  the  expres- 
sion of  these  dominant  unit  characters  must  have  negative  sur- 
vival value.  This  is  exactly  what  the  dominant  inhibitory  factor 
amounts  to;  it  is  something  which  prevents  the  expression  of  a 
dominant  unit  character.  One  is  forced  to  the  following  conclu- 
sion. Although  dominant  inhibitory  factors  may  have  come  into 
existence  just  as  frequently  and  numerously  as  the  other  types  of 
factors  and  determiners,  most  of  these  inhibitors  would  have  been 
ehminated  through  natural  selection  on  account  of  their  negative 
survival  value.  The  few  dominant  inhibitory  factors  which  per- 
sisted would  be  those  which  inhibited  "indifferent"  characters. 
Color  in  grains  of  corn  is  doubtless  an  "indifferent"  character. 

(There  are  two  types  of  things  which  are  fairly  common  and 
which  might  be  confused  with  inhibitory  factors:  (i)  simple 
determiners  which  produce  such  characters  as  to  "mask"  other 
characters  without  really  inhibiting  other  determiners;  (2)  "lethel 
factors"  which  will  be  explained  on  p.  69.) 

On  the  other  hand,  it  is  not  surprising  to  find  that  supple- 
mentary factors  are  rather  common.  The  supplementary  factor 
carries  further  a  reactioh  which  has  been  brought  up  to  a  certain 
point  by  some  other  force.  Exactly  this  sort  of  mechanism  must 
play  a  large  part  in  the  ontogeny  of  most  organisms.     The  litera- 


The  Factor  Hypothesis  69 

ture  of  genetics  is  full  of  such  things  as  "intensifying"  factors, 
''diluting"  factors,  and  "(listril)ution"  factors,  all  of  which  follow 
the  supplementary  factor  mechanism. 

Complementary  factors  are  also  common,  quite  as  we  should 
expect.  Any  complex  machine  contains  numerous  parts,  capable 
of  applying  numerous  dififerent  forces,  each  one  of  which  may  be 
quite  functionless  in  itself,  but,  in  interaction  with  some  of  the 
others,  will  produce  a  visible  result.  The  living  organism  is  just 
such  a  complex  machine. 

It  is  not  necessary  that  complementary  factors  exist  only  in 
pairs.  In  corn  itself  there  is  an  additional  complementary  factor 
A  for  color  of  grain  (Emerson  4).  R,  C,  and  A  must  all  three  be 
present  for  the  grain  to  have  color.  (A  cross  between  the  red 
type  AARRCC  and  the  white  type  aarrcc  would  give  a  red  Fi, 
AaRrCc,  and  an  F2  which  would  show  a  ratio  of  27  reds  137  whites.) 
Many  sizable  sets  of  complementary  factors  doubtless  exist  in  the 
organism  without  our  knowing  it.  One  can  of  course  never  iden- 
tify a  factor  wdthout  discovering  a  race  in  which  this  factor  is 
lacking.  Where  the  character  involved  is  an  "indifferent"  one, 
races  lacking  one  of  the  complementary  factors  are  frequently 
discoverable.  Where,  however,  the  character  involved  is  vital 
to  the  existence  of  the  organism,  it  is  impossible  to  discover  a  race 
lacking  one  of  the  necessary  complementary  factors,  since  such  a 
race  cpuld  not  live.  In  this  connection  a  word  should  be  said  of 
"lethal"  factors. 

Lethal  factors,  of  which  a  surprisingly  large  number  have  been 
identified  during  the  last  few  years  of  investigation,  are  by  defini- 
tion factors  which  bring  death  to  the  organism.  One  might 
conclude  from  such  a  definition  that  these  are  inhibitory  factors 
which  are  inhibiting  some  vital  function  of  the  organism,  but  this 
is  practically  never  the  case.  In  almost  all  cases,  it  is  the  homozy- 
gous recessive  condition  only  which  brings  the  lethal  eft'ect,  so 
that  it  is  really  the  absence  of  the  factor  that  is  lethal  rather  than 
the  factor  itself.  How  are  we  to  interpret  this  behavior  in  terms 
of  the  mechanisms  which  we  have  already  described  ?  One  might 
assume  that  merely  a  simple  determiner  is  involved,  a  determiner 
for  some  vital  function,  so  that  its  absence  brings  the  lethal  eft'ect. 
On  the  other  hand,  it  is  altogether  likely  that  more  than  one  gene 


yo  Outline  of  Genetics 

is  necessary  to  the  success  of  this  vital  function.  On  such  a  basis, 
the  lethal  would  be  regarded  as  a  factor  rather  than  a  determiner, 
and  would  of  course  be  thrown  into  the  complementary  factor 
class.  The  present  writer  does  not  believe  that  genes  should  be 
called  factors  simply  because  there  theoretically  may  exist  other 
genes  necessary  to  the  production  of  the  character  in  question; 
but  holds  the  view  that  the  term  "determiner"  should  be  main- 
tained so  long  as  only  one  of  the  effective  genes  in  the  set  has  been 
identified.  In  the  case  of  the  lethals,  however,  there  have  been 
discovered  in  the  same  organism  (the  fruit  fly)  a  number  of  genes, 
the  absence  of  any  one  of  which  will  bring  the  lethal  effect.  It 
is  reasonable,  therefore,  to  regard  these  as  composing  one  or  more 
complementary  sets  governing  the  performance  of  certain  vital 
functions. 

(The  student  may  wonder  how  it  is  possible  to  identify  lethal 
factors  when  their  absence  simply  brings  death,  an  unrecordable 
phenotype  in  the  population.  This  will  be  understood  later  when 
the  subject  of  "linkage"  is  discussed.) 

It  should  be  realized  that  genes,  be  they  factors  or  deter- 
miners, may  at  times  have  more  than  one  role.  The  A  factor  in 
corn,  mentioned  above,  interacts  with  R  and  C  in  a  complementary 
set  for  the  production  of  aleurone  color  in  the  grain.  A  also  has 
an  effect  in  producing  pigment  in  the  vegetative  parts  of  the  plant. 
The  R  and  C  factors  in  stocks  (Saunders  6)  are  a  complementary 
pair  for  the  production  of  colored  flowers.  R,  C,  and  a  third 
factor,  H,  must  all  be  present  for  there  to  be  hairs  on  the  leaves. 
Such  phenomena  support  the  belief  that  the  gene  is  not  a  "vital- 
istic"  unit  endowed  with  a  specific  function  in  connection  with  a 
single  plant  character,  but  rather  is  of  the  nature  of  many  chemi- 
cals, the  presence  of  which  will  inevitably  affect  the  course  of 
more  than  one  type  of  reaction. 

LITERATURE  CITED 

1.  Bateson,  W.,  Mendel's  laws  of  heredity.     Cambridge.  1909. 

2.  CzAPEK,  P.,  and  M.  E.,  Biochemie  der  Pflanzen.     Jena.  1913. 

3.  East,  E.  M,,  and  Hayes,  H.  K.,  Inheritance  in  maize.     Conn. 
Agric.  Exper.  Sta.  Bull.  no.  167.     pp.  142.  ph.  25.  191 1. 


The  Factor  IlypotJiesis  71 

4.  Emerson,  R.  A.,  A  fifth  pair  of  factors,  Aa,  for  alcuronc  color 
in  maize,  and  its  relation  to  the  Cc  and  Rr  pairs.  Cornell 
Univ.  Agric.  Exp,  Sta.     iMem.  16.  pp.  231-289.   1918. 

5.  LoTSY,  J.  P.,  La  theorie  du  croisement.  Arch.  Xeerland  Sci. 
Exact,  et.  Nat.  Ill  B  2: 1-61.  1914. 

6.  Saunders,  E.  R.,  Further  contribution  to  the  study  of  the 
inheritance  of  hoariness  in  stocks  {Matthiola).  Proc.  Roy. 
Soc.  B  85.  1912. 


CIL\PTER  V 

INHERITANCE  OF  QUANTITATIVE 
CHARACTERS 

This  phase  of  the  factor  hypothesis,  if  true,  is  of 
fundamental  importance,  not  only  to  genetics  but  to 
general  biology.  It  is  based  upon  the  conception  of 
cumulative  factors,  and  as  it  is  presented  it  will  be  real- 
ized that  it  throws  light  not  only  upon  numerous  breed- 
ing problems,  but  also  upon  variation  in  general,  which 
means  evolution  also.  A  cumulative  factor  may  be  de- 
fined as  one  which,  when  added  to  another  similar  factor, 
affects  the  degree  of  development  of  a  character. 

It  will  be  recalled  that  Correns  crossed  red  and  white 
strains  of  Mirahilis  and  obtained  pink  hybrids.  The 
suggested  explanation  of  this  result  was  that  a  single 
dose  of  the  red  determiner  gives  pink  while  a  double 
dose  gives  red.  When  Correns  inbred  these  pink 
hybrids,  he  obtained  the  result  presented  in  fig.  8,  that 
is,  I  red  12  pink:i  white.  The  mechanism  in  this  case 
is  quite  evident. 

With  this  diagram  in  mind  we  shall  consider  some  of 
the  experiments  of  Nilsson-Ehle  (6,  7)  at  the  Swedish 
Experiment  Station.  He  crossed  two  strains  of  wheat 
with  red  and  white  kernels.  The  Fj  individuals  had  light 
red  kernels,  which  of  course  suggests  a  repetition  of  the 
situation  shown  by  Mirahilis  in  the  experiment  of  Cor- 
rens. The  F2  generation,  however,  showed  a  very  dif- 
ferent result.     The  reds  and  whites  appeared  in  the 

72 


Inheritance  of  Quanlilative  Characters  73 

ratio  of  15:1;  but  in  addition  to  this,  among  the  15  reds 
there  could  be  distinguished  varying  degrees  of  redness. 
Nilsson-Ehle  suspected  that  the  15:1  meant  a  dihybrid 
ratio,  16  individuals  being  necessary  to  give  the  ratio;  so 
he  constructed  the  tentative  scheme  shown  in  fig.  10. 

This  shows  a  regular  dihybrid  ratio,  except  that  the 
two  factors  in^'olved  are  similar.  Applying  the  single 
dose  and  double  dose  conception,  as  used  in  the  case  of 
Corren's  pink  Mirabilis,  we  reach  the  following  con- 
clusions: only  no.  i  has  four  doses  and  therefore  it  is 
deep  red;  nos.  2,  3,  5,  and  9  have  three  doses  and  are 
somewhat  lighter  red;  nos.  4,  6,  7,  10,  11,  and  13  have 
two  doses  and  are  still  lighter  red;  nos.  8,  12,  14,  and 
15  have  one  dose  and  are  very  light  red;  while  no.  16 
alone  has  no  dose  and  is  the  only  pure  white.  This 
accounts  for  the  15:1  ratio,  and  the  different  shades  of 
red.  This  is  of  course  quite  in  accord  with  the  Mendelian 
method  of  inheritance,  only  two  assumptions  being 
necessary:  (i)  that  dominance  is  absent,  two  doses 
having  twice  the  effect  of  one;  (2)  that  the  independent 
similar  factors  are  cumulative  in  their  operation.  This 
was  Nilsson-Ehle's  conception,  and  of  course  he  tested 
it  by  further  experimental  work,  the  results  consistently 
confirming  his  assumptions. 

Since  it  is  important  to  fix  this  conception  clearly  in 
mind,  another  type  of  diagram  may  represent  the  facts 
even  more  clearly.  The  proportion  of  the  individuals 
showing  the  various  degrees  of  redness  in  the  F2  is 
graphically  recorded  in  fig.  11,  each  dot  representing 
one  dose  of  the  factors  in  question. 

Continuing  these  investigations,  Nilsson-Ehle  next 
discovered  a  new  strain  of  red-grained  wheat,  which. 


74 


Outline  of  Genetics 


when  crossed  with  the  pure  white  strain,  yielded   Fi 
hybrids  of  an  intermediate  degree  of  redness  as  before. 


Fig.  io. — Diagram  illustrating  Nillson-Ehle's  explanation  of 
15: 1  ratio  obtained  in  F2  generation  from  cross  between  red-grained  and 
white-grained  wheat. 

The  F2  generation,  however,  showed  a  different  situation. 
Reds  and  whites  were  obtained  in  the  proportion  of 
63 : 1 ;    the  63  reds  as  before  falhng  naturally  into  differ- 


Inheritance  of  Qua ntita live  Characters 


75 


ent  groups  on  the  basis  of  degree  of  redness.  Applying 
the  same  conception  as  before,  Nilsson-Ehle  discov- 
ered that  in  this  case  he  was  dealing  with  a  trihybrid 
situation.  Without  constructing  the  usual  Mendelian 
checkerboard,  which  would  have  to  be  extensive  enough 


Pure  Red 


VVKite 


Grades  of  Pink 

Fig.    II. — Another  method   of   visualizing   Nillson-Ehle's    15:1 
ratio  (see  fig.  10). 

for  64  individuals,  the  situation  as  it  appeared  in  the  F2 
generation  may  be  represented  by  lig.  12.  If  this  graph 
be  surmounted  by  a  cur\'e,  we  recognize  the  regular 
"probabilities  curve,"  exactly  the  kind  used  by  biome- 
tricians  to  represent  fluctuating  variations  about  a  specific 
type. 


76 


Outline  of  Genetics 


#• 


While 


P"'^  ^^^  Intcrnicdiato  Grades 

Fig.  12. — Diagram  illustrating  Nillson-Ehle's  63:1  ratio 


I nJierilance  of  Quanlilaliue  Characters  77 

This  conception  of  cumulative  factors,  therefore, 
has  far-reaching  significance.  For  a  long  time,  biologists 
have  recognized  individual  quantitative  variation  within 
the  species.  Darwin  depended  upon  it  as  the  basis  of 
his  theory  of  evolution;  in  fact,  ever  since  Darwin's 
Origin  of  species,  individual  variation  has  been  funda- 
mental in  our  conceptions.  To  account  for  this  univer- 
sally recognized  phenomenon,  Darwix  proposed  his 
transportation  hypothesis  and  Weismann  offered  his 
germinal  selection,  both  of  which  were  unsatisfactory 
explanations.  Aside  from  these  two  attempts  to  explain 
individual  variation,  no  other  comprehensive  scheme 
had  been  presented.  Biologists  had  simply  recognized 
the  fact  of  individual  variation  without  any  clear  con- 
ception of  the  mechanism. 

The  importance  of  this  new  theory,  therefore,  is 
obvious.  It  is  an  ingenious  explanation  of  the  inherit- 
ance of  quantitative  characters  and  of  the  existence  of 
individual  variations.  Furthermore,  the  theory  has 
not  been  developed  through  meditation,  but  has  its 
basis  in  scientific  experiments.  It  is  imaginative  to  a 
certain  extent,  as  is  every  other  valuable  theory,  but 
unlike  most  such  theories,  it  has  a  substantial  foundation, 
namely,  Mendel's  law. 

The  importance  of  the  possible  role  of  cumulative 
factors  in  explaining  individual  variation,  which  in 
turn  may  be  the  basis  of  a  certain  type  of  evolution, 
has  been  emphasized  because  its  importance  has 
perhaps  not  yet  been  sufliciently  appreciated.  It 
promises  to  be  one  of  the  most  important  theories  of 
biology,  but  of  course  will  bear  further  testing  by  in- 
vestigators. 


78  Outline  of  Genetics 

The  doctrine  of  cumulative  factors  was  further  devel- 
oped by  Emerson  and  East  (5)  in  their  work  with  corn. 
They  were  able  to  explain  some  of  the  ratios  obtained 
by  assuming  three  or  four  separately  inherited  cumula- 
tive factors,  just  as  Nilsson-Ehle  had  done.  They 
obtained  other  ratios,  however,  which  required  more 
independent  cumulative  factors  to  explain.  Some  idea 
of  the  extent  of  these  investigations  may  be  gained  by 
noting  the  list  of  plant  characters  whose  inheritance  they 
explained  on  the  basis  of  cumulative  factors:  number  of 
rows,  length  of  ear,  diameter  of  ear,  weight  of  seed, 
breadth  of  seed,  height  of  plant,  number  of  stalks  per 
plant,  earliness  of  flowering.  In  all  of  these  cases 
breeding  gave  the  same  characteristic  results.  A  cross 
between  extreme  parents  gave  hybrid  progeny  inter- 
mediate as  to  the  character  in  question;  and  in  the  F2 
generation  the  two  extremes  reappeared,  along  with  all 
gradations  of  intermediates.  The  relative  frequencies 
of  these  classes  always  resembled  the  normal  probabili- 
ties curve. 

Nilsson-Ehle  had  been  able  to  put  his  F2  inter- 
mediates into  rather  definite  classes,  corresponding  to 
the  number  of  doses  of  the  determiner  each  had  received. 
Emerson  and  East,  however,  could  not  do  this  with 
such  exactness.  Their  results  showed  all  gradations, 
but  they  could  not  distinguish  any  definite  groups; 
that  is,  gradation  was  continuous  and  complete.  In 
other  words,  they  could  not  tell  with  certainty  from  out- 
ward appearance  just  how  many  doses  of  cumulative 
factors  an  individual  contained.  Their  results,  there- 
fore, do  not  seem  so  clear  and  striking  as  those  of  Nils- 
son-Ehle, but  they  are  by  no  means  vague  and  uncer- 


Inheritance  of  Quantitative  Characters  79 

tain.  For  example,  even  if  they  could  not  say  definitely 
that  a  certain  individual  had  three  doses,  they  could 
always  say  approximately  how  many  doses  it  had;  and 
the  breeding  results  always  confirmed  the  idea  of  a  num- 
ber of  cumulative  factors  at  work.  For  example,  a 
plant  with  three  doses  may  vary  with  respect  to  the 
character  in  question.  It  may  approach  the  condition 
of  the  plant  with  four  doses  or  it  may  vary  toward  the 
two-dose  condition.  Such  variation  may  be  explained 
by  outside  influences.  Any  classification  of  the  F2  indi- 
viduals on  the  basis  of  the  number  of  doses  is  more  or 
less  obscured  by  the  influence  of  outside  factors  which 
are  uncontrollable,  or  at  least  uncontrollable  as  yet  in 
breeding  work. 

Emerson  and  East  have  visualized  these  outside 
factors  and  discussed  them.  In  order  to  explain  this 
discussion,  however,  we  must  recall  a  feature  of  genetics 
which  has  previously  been  mentioned.  Plant  varia- 
tions in  the  largest  sense  fall  under  two  categories,  those 
due  to  (i)  differences  in  gametic  constitution,  and  (2) 
those  due  to  responses  to  environment.  The  first  cate- 
gory is  the  basis  of  all  iMendelian  conceptions,  while  the 
second  category  includes  such  variations  as  are  usually 
thought  not  to  be  inherited,  being  acquired  characters. 
This  category  is  now  commonly  called  fluctuating  varia- 
tions. 

An  illustration  will  make  these  two  categories  clear. 
Assume  that  a  plant  with  a  determiner  for  tallness  usu- 
ally becomes  6  feet,  while  one  without  this  determiner 
becomes  3  feet.  The  6-foot  plant,  however,  grown  in 
good  soil  becomes  6.5  feet,  while  in  poor  soil  it  is  5.5  feet. 
In  inheritance,  of  course,  the  6.5-  and  5.5-foot  plants 


8o 


Outliiie  of  Genetics 


behave  exactly  alike;  the  same  is  true  of  6-foot  plants. 
It  must  be  evident,  therefore,  that  a  classification  of 
F2  individuals  on  the  basis  of  the  number  of  doses  might 
well  be  slightly  obscured.  If  outside  influences  were 
lacking,  the  F2  situation  could  be  represented  by  fig.  13 ; 


Fig.  13. — Diagram  illustrating  distribution  of  phenotype  classes  in 
Yi  population  from  cross  involving  cumulative  factors.  Practically 
same  diagram  as  fig.  12,  and  interpreted  in  same  way;  short  rectangle 
at  left  indicates  that  very  few  plants  of  population  contain  maximum 
number  of  doses;  short  rectangle  at  right  indicates  that  very  few  plants 
contain  minimum  number  of  doses;  plants  with  intermediate  number  of 
doses  most  numerous,  as  indicated  by  tall  rectangle  in  middle  (see  also 
fig-  14). 

but  when  outside  influences  are  active,  it  may  be  repre- 
sented by  fig.  14.  It  will  be  seen  from  this  last  diagram 
that  not  all  individuals  belonging  to  a  particular  size 
class  may  have  the  same  number  of  doses;  that  is,  con- 
ditions surrounding  the  development  of  a  smaller-dosed 
individual  may  be  so  much  better  than  those  for  a  larger- 
dosed  individual  that  they  may  exchange  size  classes  in 
the  result.     In  this  way,  the  results  of  germinal  constitu- 


Inheritance  of  Quanlitalive  Characters 


8i 


tion  may  be  somewhat  obscured  by  the  varying  external 
conditions  of  growth. 

Another  factor  that  may  obscure  these  results  is 
what  is  called  physical  correlation.  For  example,  a 
corn  plant  of  small  size,  but  with  the  hereditary  capacity 
for  producing  large  ears,  could  not  fully  express  this 


A 


A 


K 


K 


\ 


\ 


\ 


^ 


K 


Fig.  14. — Diagram  illustrating  how  environmental  inlluences  may 
obscure  phenotype  classes  of  F2.  Overlapping  of  phenotype  classes 
makes  possible  that  two  apparently  identical  plants  might  actually  have 
a  different  number  of  doses;  diagram  also  shows  that  while  breeder 
could  not  recognize  whether  a  plant  had  two  or  three  doses,  he  could 
distinguish  between  plants  of  two  and  four  doses,  etc.  Thus  intelligent 
selection  could  be  effective. 


capacity.     It  could  not  produce  as  large  ears  as  if  it  had 
been  a  large-sized  plant. 

Such  are  some  of  the  conditions  or  factors  that  tend 
to  obscure  results  in  the  F2  generation  and  give  rise  to 
ratios  hard  to  interpret.  The  weaker  the  influence  of 
these  factors  the  more  clearly  do  the  phenomena  of 
cumulative  factors  come  out.  The  total  result  of  this 
phase  of  the  work  of  Emerson  and  East,  in  spite  of 
obscuring   conditions    that   have    arisen,    has    been    to 


82  Outline  of  Genetics 

strengthen  greatly  the  conception  of  cumulative  factors. 
A  summary  of  their  conclusions  is  stated  as  follows: 

^'When  one  is  dealing  with  quantitative  characters, 
that  is,  those  produced  by  cumulative  factors,  he  is 
confronted  by  exactly  the  same  principles  of  Mendelian 
inheritance  as  have  long  been  known  to  apply  to  quali- 
tative characters.  With  quantitative  characters,  how- 
ever, the  problem  is  more  complex,  due  chiefly  to  two 
things:  (i)  we  are  usually  deahng  with  more  factors, 
and  factors  cumulative  in  their  operation;  (2)  the  signifi- 
cance of  the  breeding  results  is  usually  somewhat 
obscured  by  the  natural  fluctuations  due  to  response  to 
uncontrollable  factors  in  the  environment." 

In  connection  with  the  cumulative  factor  concept,  a 
modifying  statement  should  be  made  as  to  the  mecha- 
nism involved.     Heretofore  it  has  been  assumed   that 
we    are    dealing   with    numerous,    separately    inherited 
factors,  absolutely  identical  in  their  nature,  cumulative 
in  their  effect.     No  doubt  one  might  regard  with  sus- 
picion such  a  seemingly  artificial  mechanism.     Probably 
it  would  be  easier  to  believe  if  it  were  modified  in  the 
following  manner.     Instead  of  assuming  that  the  numer- 
ous factors  are  identical  in  function,  we  may  assume 
that  each  of  these  factors  has  its  own  peculiar  function, 
but  that  that  function  plays  a  part,  directly  or  indirectly, 
in  developing  the  quantitative  character  in  question. 
For  example,  suppose  height  is  the  character.     One  of 
the  factors  determines   the  development  of  numerous 
nodes;    another   increases    the   amount   of  chloroj)hyll ; 
another  determines  the  size  and  \'igor  of  the  root  system; 
another  brings  early  germination  and  a  long  growing 
season.     Such  factors,   although  not  identical,  will  be 


Inheritance  of  Quantitative  Characters  83 

cumulative  in  increasing  the  height  of  the  plant.  Of 
course  a  single  dose  of  one  type  of  factor  may  not  bring 
the  same  increase  in  height  as  would  a  single  dose  of  one 
of  the  other  types,  and  therefore  the  mathematics  of  the 
situation  will  be  slightly  modified.  The  fundamental 
mathematical  system,  however,  will  remain  the  same, 
and  we  will  have  the  satisfaction  of  dealing  with  a  natural 
mechanism  rather  than  an  artificial  one. 

A  few  of  the  applications  of  the  cumulative  factor  hypothesis 
are  worth  considering.  Assume  that  a  practical  breeder  crosses 
two  extreme  parent  types  in  the  hope  of  obtaining  a  hybrid  com- 
bining the  desirable  characters  of  the  two  parents.  If  the  material 
is  corn,  he  might  use  one  parent  with  large  grains  but  few  in  num- 
ber, while  the  other  parent  had  many  grains  but  small  ones.  Such 
quantitative  characters  as  these  would  be  determined  by  cumula- 
tive factors,  and  the  hybrid  would  be  intermediate  with  respect 
to  both  of  these  characters,  that  is,  the  grains  would  be  of  medium 
size  and  medium  number.  No  matter  how  many  crosses  he  made, 
he  would  always  get  this  result,  and  not  the  desired  combination 
of  large  grains  and  many  of  them. 

Suppose  now  that  these  intermediate  hybrids  are  inbred  in  the 
hope  of  obtaining  the  desired  combination  among  the  individuals 
of  the  F2  generation.  It  will  be  realized  that  the  chances  of  obtain- 
ing a  plant  combining  the  two  extreme  characters  of  large  grains 
and  numerous  grains  would  depend  upon  the  number  of  factors 
that  enter  into  the  make-up  of  these  quantitative  characters. 
Assume  that  there  are  five  factor  pairs  in  each  case.  The  mathe- 
matics of  the  situation  would  show  that  in  order  to  get  the  desired 
pure  type  from  a  cross  between  two  parents,  each  having  their 
desirable  character  determined  by  five  pairs  of  cumulative  factors, 
it  would  require  100  acres  of  corn  to  have  an  even  chance  of  getting 
one  such  individual  in  the  Fj  generation.  It  is  altogether  unlikely 
that  any  farmer  would  use  100  acres  and  a  corresponding  amount 
of  labor  on  such  an  extreme  chance.  Even  an  agricultural  experi- 
ment station  would  not  feel  justified  in  conducting  such  an  experi- 
ment. 


84  Otitline  of  Genetics 

The  question  arises  whether  there  is  any  way  of  avoiding  this 
impossible  situation.  The  escape  is  suggested  by  the  fact  that 
time  can  take  the  place  of  numbers.  East  has  shown  that  by 
growing  looo  individuals  in  the  F2  generation,  100  in  the  F3,  and 
50  in  the  F4,  one  stands  as  much  chance  of  getting  the  desired 
combination  as  by  growing  250,000  in  the  F2,  provided  an  intelli- 
gent selection  is  made  in  each  generation.  In  other  words,  one 
who  understands  the  mechanism  of  the  inheritance  of  quantita- 
tive characters  will  grow  only  1000  individuals  in  the  F2  genera- 
tion, and  will  select  for  seed  only  those  individuals  with  the  most 
favorable  combination  of  factors.  In  this  way,  by  intelligent 
selection,  factors  are  'Spiled  up"  in  the  right  direction  from  year 
to  year.  In  a  few  years  the  desired  result  will  be  reached  without 
the  necessity  of  ever  growing  a  very  large  number  of  individuals. 
Such  work  is  practicable  at  experiment  stations,  and  it  is  the  kind 
of  work  that  a  number  of  them  have  been  doing.  Even  the  farmer 
is  able  to  accomplish  this.  Although  his  selection  of  individuals 
is  not  quite  as  intelligent  as  that  of  a  scientific  breeder,  he  is  at 
least  selecting  in  the  right  direction  and  making  some  advance. 
A  little  more  time  and  a  little  more  acreage  would  bring  him  very 
close  to  the  desired  goal. 

A  further  application  of  the  cumulative  factor  hypothesis 
may  be  considered.  The  practice  we  have  been  discussing  under 
the  title  of  "inheritance  of  quantitative  characters"  seems  to  be 
little  more  than  what  has  already  been  called  artificial  selection, 
which  is  the  oldest  of  all  methods  of  plant  breeding.  It  is  a  method 
that  was  thought  to  be  discredited  entirely  by  the  work  of  De  Vries 
and  JoHANNSEN  when  they  discovered  ''elementary  species"  or 
''pure  lines,"  and  demonstrated  that  artificial  selection  could  never 
result  in  any  large  or  permanent  improvement.  In  consequence 
of  this,  artificial  selection,  as  the  most  important  method  of  secur- 
ing desirable  races,  gave  place  to  pedigree  culture  at  a  number  of 
experiment  stations.  The  older  method  was  not  entirely  aban- 
doned, for  it  had  its  uses,  but  many  regarded  it  as  a  medieval 
method  of  breeding.  The  artificial  selection  which  we  have  been 
describing,  however,  is  distinctly  different  from  the  method  prac- 
ticed by  the  early  breeders.  In  brief  statement,  the  difference  is 
as  follows. 


Inheritance  of  Quantitative  Characters  85 

The  selection  proposed  is  preceded  by  an  intelligent  hybridiz- 
ing, and  after  that  genotypes  rather  than  phenotypes  are  se- 
lected; that  is,  the  selection  is  made  on  the  basis  of  germ  plasm 
rather  than  body  plasm.  'J'his  would  be  a  sufficient  reason  for 
the  superiority  of  the  new  method  of  artiiicial  selection  as  compared 
with  the  old.  A  little  further  analysis  will  make  the  difference 
clearer. 

In  the  old  method  of  artificial  selection,  the  breeder,  in  the 
first  place,  is  dealing  with  such  germinal  variations  as  happen  to 
appear  in  his  crop;  and,  in  the  second  place,  he  is  dealing  with 
those  fluctuations  which  appear  as  responses  to  the  environment. 
When  he  selects  a  large  plant  to  use  for  seed,  that  plant  may  be 
large  on  account  of  its  germinal  constitution;  but,  on  the  other 
hand,  it  may  be  large  because  it  is  growing  in  a  less  crowded  place 
or  a  place  more  heavily  fertilized  than  the  others.  In  that  case, 
the  large  plant  might  not  furnish  good  seed.  The  plant  breeder 
of  the  old  method  undoubtedly  made  such  unfortunate  selections 
frequently;  that  is,  he  selected  on  the  basis  of  external  appearance, 
and  external  appearance  is  very  often  a  poor  index  of  hereditary 
capacity.  Furthermore,  he  would  not  keep  his  lines  pure,  but 
would  deal  constantly  with  an  unmanageable  mixture  of  good  and 
mediocre  types.  Intelligent  selection  is  based  on  germinal  con- 
stitution only^by  keeping  careful  pedigree  records  a  selection  of 
genotypes  is  possible — and  therefore  its  results  are  quicker  and 
surer.  It  is  really  a  pedigree  culture  rather  than  a  mass  culture 
method. 

Another  phase  of  the  subject  should  be  considered.  When  a 
plant  breeder  is  trying  to  improve  his  crops  by  selection  for  quan- 
titative characters,  although  he  uses  the  old  method  of  selection, 
he  is  likely  to  be  making  some  gain,  as  the  experience  of  hundreds 
of  years  has  shown.  The  germinal  constitution  of  his  crop  plants 
is  masked  by  fluctuations,  of  course,  but  this  mask  is  not  complete. 
Most  of  the  plants  he  selects  are  bound  to  possess  high  numbers 
of  factors  of  the  right  kind,  and  he  probably  rejects  most  of  the 
plants  with  few  factors.  In  any  event,  he  has  generally  succeeded 
in  the  long  run  in  getting  a  somewhat  improved  race. 

A  summarized  statement  of  this  situation  may  be  helpful. 
Our  recently  developed  knowledge  of  the  inheritance  of  quanti- 


86  Outline  of  Genetics 

tative  characters  seems  to  justify  artificial  selection,  but  it  does 
not  justify  the  old  blind  method  of  selection.  It  emphasizes  the 
need  of  intelligent  selection,  and  shows  how  such  selection  can  be 
made.  In  order  to  do  this,  one  must  understand  the  mechanism 
of  inheritance  involved,  and  must  make  his  selection  on  the  basis 
of  genotype  rather  than  phenotype.  All  along  the  line,  strains 
must  be  discarded  which,  though  recommended  by  the  phenotype 
of  one  of  their  ancestors,  are  to  be  condemned  on  the  basis  of  their 
breeding  capacity.  Selection  is  always  to  be  made  on  the  basis 
of  breeding  capacity,  that  is,  genotype. 

The  situations  just  considered  enable  one  to  understand  two 
phenomena  which  have  been  baffling  scientific  plant  breeders  for 
some  years.  The  races  of  plants  improved  by  artificial  selection 
have  usually  reverted  to  type  when  selection  ceases.  This  fact 
was  recognized  for  a  long  time,  but  was  first  pointed  out  clearly 
by  De  Vries  (3) .  Since  then  we  have  always  expected  this 
result,  that  no  improvement  will  maintain  itself,  but  will  run  back 
unless  the  selection  is  continuous.  When  a  practical  breeder 
announces  that  he  has  developed  by  selection  a  new  race  which 
continues  to  breed  true  without  further  selection,  we  are  inclined 
to  disbelieve  him,  for  we  know  that  only  elementary  species  breed 
true.  We  explain  that  the  practical  breeder  bases  his  selection 
on  fluctuations,  and  therefore  his  new  race  is  bound  to  revert  to 
type.  It  is  obvious  now  that  there  is  a  flaw  in  this  argument. 
The  practical  breeder  may  be  basing  his  selection  on  fluctuations, 
but  at  the  same  time  he  may  be  piling  up  cumulative  factors  in 
the  right  direction.  Thus  he  might  eventually  secure  a  race  con- 
taining all  the  cumulative  factors.  Such  a  race  would  be  a  homo- 
zygote  and  could  not  help  breeding  true.  Most  of  the  claims  of 
artificially  improved  races  that  breed  true  may  be  false,  but  it 
should  be  remembered  that  such  a  thing  is  possible,  and  may  be 
"stumbled  upon  accidentally,"  even  with  unscientific   breeding. 

There  is  another  phenomenon  which  has  been  much  discussed, 
and  which  can  now  be  explained  in  the  same  way.  This  is  the 
so-caUed  "fixation  of  hybrids."  For  years  breeders  have  made 
promiscuous  crosses  and  then  begun  artificial  selection  with  the 
F2  generation.  Eventually  they  have  secured  a  pure-breeding 
new  type.     It  will  be  remembered  that  it  was  in  this  way  that 


Inheritance  of  Quantitative  Characters  87 

East  worked  with  the  quantitative  characters  in  corn,  and  the 
explanation  is  the  same. 

In  addition  to  the  practical  value  of  the  concei)tion  of  cumu- 
lative factors,  the  theoretical  value  is  worth  considering,  for  it 
explains  things  that  have  been  very  vaguely  understood.  This 
conception  suggests  that  the  origin  of  species  by  natural  selection 
in  the  way  described  by  Darwin,  a  method  which  for  some  time 
has  been  thought  impossible,  may  actually  be  possible  within  limits. 

Of  course  natural  selection  in  a  certain  sense  has  always  been 
accepted,  almost  as  generally  as  the  fact  of  evolution.  The  point 
in  dispute  is  as  follows.  Darwin  used  as  the  basis  of  natural 
selection  those  small  individual  variations  which  we  have  come  to 
call  fluctuations,  the  same  kind  of  variations  the  old  plant  breeder 
used  in  his  artificial  selection.  Darwin  claimed  that  such  varia- 
tions could  be  piled  up  until  the  result  would  be  a  new  species.  It 
was  in  1900  that  De  \'ries  showed  in  convincing  way  that  this 
kind  of  variation  never  resulted  in  a  new  species;  at  best  it  only 
developed  a  race  which  approached  the  boundary  of  the  species 
and  never  crossed  it.  Moreover,  such  a  race  would  revert  to 
type  rapidly  as  soon  as  some  slight  change  in  conditions  set  up  a 
new  standard  for  selection.  This  argument,  confirmed  by  experi- 
ment, has  been  generally  accepted. 

We  now  know  that  individual  variations  are  not  always  mere 
fluctuations  or  responses,  but  may  be  due  to  varying  doses  of 
cumulative  factors.  A  selection  on  this  basis  may  very  well  result 
in  a  new  race  that  breeds  true;  and  a  race  that  breeds  true  is 
De  Vries'  definition  of  a  new  species.  To  reestablish  Darwin's 
ideas  on  the  origin  of  species  is  certainly  an  important  considera- 
tion. The  situation  illustrates  how  genetics  and  evolution  are 
tied  up  together,  so  that  neither  one  of  them  can  be  appreciated 
fully  without  some  knowledge  of  the  other. 

A  few  words  may  be  said  in  reference  to  the  reversion  of  an 
old  race  to  its  original  specific  type.  De  \'ries  outlined  the 
situation  clearly,  and  his  conclusions  are  generally  accepted.  It 
is  doubtful,  however,  whether  it  has  ever  been  understood,  since 
no  one  has  ever  devised  a  reasonable  mechanism  for  such  a  rever- 
sion. The  conception  of  cumulativ'e  factors  supplies  this  mecha- 
nism.    A  new  race,  developed  by  natural  or  artificial  selection 


88  Outline  of  Genetics 

among  individual  differences,  means  the  piling  up  of  cumulative 
factors  in  a  given  direction.  Stop  the  selection  and  the  old  plants 
with  the  small  numbers  of  factors  are  allowed  to  survive,  reproduce, 
cross  with  the  others,  and  eventually  bring  back  the  species  to  the 
original  average  condition. 

One  very  seldom  has  any  occasion  to  work  out  problems  on 
cumulative  factors,  since  here  the  phenotypes  do  not  show  up  as 
clearly  as  they  do  in  connection  with  the  other  factor  types.  Any 
such  problems,  however,  could  readily  be  solved  by  some  such 
method  as  the  following.  Remember  that  we  are  dealing  with  a 
dominance  absent  situation;  and  represent  the  number  of  doses 
as  exponents  attached  to  the  numbers  which  indicate  the  frequen- 
cies of  the  different  classes. 

AaBhCcXAABbcc. 

The  A  set  gives  a  ratio  of  i  with  two  doses :  i  with  one  dose,  and 
should  be  represented  as  i^:  i'. 

The  B  set  gives  i  with  two  doses:  2  with  one  dose:  i  with  no 
dose,  and  should  be  represented  as  12:2^:1°.  (i^*:  iOX(i^:2':  1°) 
equals  14:33  32:1^ 

The  C  set  gives  i  with  one  dose:i  with  no  doses,  or  1^:1". 
(14:33:32: 1')  x(i':i°)  equals  15:44:63:42;  I^  The  final  result  is  i 
with  five  doses: 4  with  four: 6  with  three: 4  with  two:i  with  one. 

During  the  last  decade,  the  mechanism  of  cumulative  factors 
has  been  invoked  to  explain  a  great  many  of  the  phenomena  of 
genetics.  One  noted  instance  of  this  will  be  worth  considering,  as 
it  has  a  very  important  bearing  upon  one  of  the  fundamental  con- 
cepts in  connection  with  the  mechanism  of  inheritance. 

A  few  years  ago  geneticists  might  have  been  grouped  into  two 
schools:  "mutationists,"  who  beUeved  in  the  introduction  of  new 
hereditary  units  by  mutation  alone,  maintaining  that  the  hereditary 
genes  were  invariable  and  could  not  be  modified  by  selection;  and 
''selectionists,"  who  beheved  that  the  genes  could  be  modified  by 
selection.  The  most  prominent  figure  among  the  selectionists 
was  Castle,  and  the  main  experimental  evidence  upon  which  he 
based  his  view  was  as  follows. 

Castle  (i)  isolated  a  race  of  rats  which  had  a  black  and  white 
coat  pattern  known  as  "hooded"  (the  black  pigmented  area  hav- 


Inheritance  of  Quantitative  Characters  89 

ing  the  location  and  general  shape  of  a  hood).  This  hooded  pat- 
tern bred  approximately  true  and  behaved  as  a  simple  IMendelian 
recessive  in  crosses  with  rats  of  the  ''wild"  type.  These  facts 
naturally  led  Castle  to  beUeve  that  hooded  was  a  simple  Men- 
delian  unit  character,  represented  in  the  germ  plasm  by  a  single 
gene. 

Castle  then  commenced  selection.  For  twelve  generations 
selections  were  made  from  this  new  race  without  a  single  outcross, 
that  is,  every  generation  was  inbred  (brother  and  sister  matings) , 
thus  insuring  the  constant  purity  of  the  stock.  In  one  series 
selection  was  made  for  an  increase  in  the  extent  of  the  pigmented 
areas;  in  another  series  selection  was  made  for  decrease  in  the 
extent  of  these  areas.  The  result  was  that  the  areas  in  the  one 
series  steadily  increased,  while  in  the  other  they  steadily  decreased. 
Castle  pointed  out  that:  (i)  with  each  selection  the  amount  of 
regression  ("running  back")  grew  less;  that  is,  the  effects  of 
selection  became  more  permanent;  in  other  words,  in  each  suc- 
ceeding generation  there  was  a  decreasing  tendency  to  revert  to 
the  original  average  type;  (2)  advance  in  the  upper  limit  of  varia- 
tion was  attended  by  a  like  advance  of  the  lower  limit.  The 
total  range  of  variation,  therefore,  was  not  materially  changed, 
but  there  was  a  progressive  change  in  the  point  about  which  the 
variation  occurred.  In  other  words,  it  was  like  the  progressive 
shifting  of  the  center  of  a  circle;  the  diameter  of  the  circle  did  not 
change  but  the  position  of  the  circle,  determined  of  course  by  its 
center,  was  gradually  changing.  These  were  the  two  important 
facts  which  Castle  brought  out  and  they  have  been  stated  approx- 
imately in  Castle's  own  words. 

Fig.  15  will  help  make  the  situation  clear.  The  average 
amount  of  variation  in  any  one  generation  of  the  pure  stock  (the  di- 
ameter of  the  circle  referred  to)  is  indicated  by  ,^-£->.  Of  course, 
even  "pure  stock"  varies  somewhat,  since  no  two  individuals  are 
exactly  alike,  biology  recognizing  what  is  called  "individuality." 
The  point  is  that  the  comparatively  small  variation  in  a  pure  stock 
is  not  due  to  germinal  differences,  but  to  responses  called  out  by 
varying  external  conditions,  such  as  nutrition,  light,  etc.  These 
response  variations,  usually  called  tluctuations,  vary  with  different 
individuals,  but  the  hereditary  capacity  of  all  of  them  remains 


go 


Outline  of  Genetics 


the  same.  A  selection  on  the  basis  of  fluctuations  within  a  pure 
line,  therefore,  should  not  result  in  any  permanent  improvement ; 
in  fact,  it  has  been  demonstrated  many  times  that  no  such  improve- 
ment can  be  effected  in  this  way.     When   selection   is   made, 


-  Pure-Bred 
Generations 


/         Selection 
Begins 


The  Control 

Fig.  15. — Diagram  illustrating  Castle's  selection  experiment  with 
hooded  rats. 

however,  among  varying  doses  of  cumulative  factors,  an  entirely 
different  situation  is  faced,  for  in  such  a  case  we  are  not  dealing 
with  a  pure  line. 

The  significance,  therefore,  of  Castle's  results  may  be  real- 
ized. He  bred  his  original  pure  line  for  many  generations  and 
found  that  it  varied  only  within  very  narrow  limits;    and  these 


Inheritance  of  Qiianiilatlve  Characters  91 

slight  variations  he  regarded  as  mere  lluctuations.  I-'urthermore, 
he  found  that  the  character  of  his  pure  Hne  behaved  in  crossing  as 
a  simple  unit  character  and  that  no  complex  factors  were  involved. 
With  this  evidence  he  should  not  have  been  able  to  effect  any 
permanent  changes  by  selection,  but  this  is  exactly  what  he  did. 
Selecting  in  opposite  directions,  he  developed  two  new  strains, 
the  boundaries  of  the  new  strains  being  distinct  from  one  another 
and  distinct  from  the  boundaries  of  the  original  strain,  that  is, 
the  non-selected  type  that  he  started  with. 

Castle's  next  step  was  significant.  He  crossed  each  of  his 
new  strains  with  the  same  wild  race,  the  result  being  that  each  of 
his  new  strains  behaved  as  a  simple  and  distinct  recessive  unit. 
The  high  pigmentation  strain  ''came  out  of  the  cross"  with  the 
characteristic  high  pigmentation;  the  low  pigmentation  strain 
came  out  with  the  characteristic  low  pigmentation. 

The  conclusion  from  this  series  of  experiments  may  be  given 
in  Castle's  words,  as  follows:  ''The  conclusion  seems  to  me 
unavoidable  that  in  this  case  selection  has  modified  steadily  and 
permanently  a  character  unmistakably  behaving  as  a  simple  Men- 
delian  unit."  The  importance  of  this  conclusion  is  evident. 
Mendelism  had  been  based  upon  the  conception  that  unit  char- 
acters could  not  be  modified.  Mendelians  of  the  "mutationist" 
school  had  granted  only  two  possible  methods  for  the  origin  of 
new  races:  (i)  by  recombinations  of  existing  characters  through 
hybridizing;  (2)  by  the  sudden  and  complete  dropping  out  of  an 
existing  unit  or  the  equally  sudden  addition  of  a  new  unit,  both 
of  which  possibilities  might  arise  from  mutation.  No  "mutation- 
ist" would  grant,  however,  the  possibility  of  modifying  an  existing 
unit  character,  the  thing  which  Castle  claimed  to  have  done, 
basing  his  claim  upon  well-controlled  experimental  breeding.  If 
Castle's  contention  were  true,  it  would  result  in  the  fundamental 
modification  of  Mendel's  law.  The  whole  mechanism  would  have 
to  be  modified  to  take  into  account  new  fields  of  variation  that  had 
not  been  thought  to  exist. 

The  statements  of  the  "mutationists"  in  reference  lo  these 
experiments  should  be  considered.  They  attempted  to  explain 
Castle's  results  through  the  cumulative  factor  mechanism. 
The  claim  was  made  that  Castle  had  started  with  a  character 


92  Outline  of  Genetics 

that  had  fluctuated  continually,  never  having  been  brought  to  as 
small  a  variability  as  have  most  other  characters.  The  question 
was  raised  whether  Castle's  assumption  that  this  variability  was 
merely  due  to  fluctuation  was  altogether  justified.  Might  not 
the  variability  have  been  due  to  varying  doses  of  cumulative 
factors?  Suppose  for  the  moment  that  this  were  the  case;  it 
would  not  be  surprising  that  Castle  could  develop  two  diverse 
strains  by  selection,  for  selection  would  result  in  piling  up  the 
cumulative  factors  in  one  direction  or  the  other.  Castle's 
rejoinder  was  that  if  this  were  a  cumulative  factor  situation,  why 
had  none  of  the  extremes  appeared  in  the  non-selected  stock, 
which  instead  bred  approximately  true  ?  The  answer  was  made 
that  the  extremes  did  not  appear  in  the  pure  bred  stock  merely 
because  of  the  mathematical  limitations.  If  one  is  deahng  with 
six  cumulative  factors,  and  the  so-called  pure  stock  has  an  inter- 
mediate number  of  doses,  there  could  not  be  much  chance  of 
getting  out  the  extremes  in  a  single  generation.  It  would  be 
necessary  to  secure  over  4000  progeny  to  have  an  even  chance  of 
getting  one  such  extreme;  or  about  50  progeny  to  get  anything 
that  would  very  noticeably  approach  the  extreme.  It  would  seem, 
therefore,  that  Castle's  chances  to  determine  this  would  be  very 
small.  Rats  certainly  do  not  produce  4000  progeny  in  a  single  gen- 
eration; in  fact,  they  produce  much  less  than  50;  therefore  Cas- 
tle's "pure  stock  "  went  on  in  the  intermediate  condition,  and  only 
by  selection  could  he  pile  up  the  factors  and  reach  either  extreme. 
Thus  far  the  explanation  seemed  satisfactory.  Castle 
showed,  however,  that  the  coat  pattern  condition  behaved  in 
crosses  as  a  simple  Mendelian  unit;  that  is,  it  did  not  split  up  into 
complex  ratios,  but  came  out  as  a  recessive  in  a  regular  3 :  i  ratio. 
This  really  involved  no  difliculty.  Suppose  Castle  crosses  one 
of  his  pure  strain  rats  having  the  hooded  character  with  another 
race  that  has  some  pattern  character  that  conceals  the  hooded 
character.  If  this  other  character  is  a  .simple  Mendelian  one,  the 
result  of  the  cross  would  be  the  ordinary  monohybrid  ratio;  that 
is,  in  the  F2  generation  from  such  a  cross  the  ratio  of  hooded  to 
non-hooded  (with  the  " hood-conceahng "  character)  would  be  1:3, 
which,  in  fact,  is  exactly  what  Castle  got.  At  the  same  time, 
the  amount  of  pigmentation,  determined  by  numerous  cumulative 


Inheritance  of  Quantitative  Characters  93 

factors,  might  go  on  in  the  same  intermediate  condition,  unaffected 
by  the  cross.  The  rehition  of  pattern  to  non-pattern  is  merely  a 
simple  monohybrid  system  temporarily  superimposed  upon  the 
other  more  complex  system  without  permanently  affecting  it,  any 
more  than  any  inhibitory  factor  permanently  affects  the  factors 
it  inhibits,  or  a  dominant  permanently  affects  a  recessive. 

It  was  in  this  way  that  the  mutationists  attempted  to  explain 
away  Castle's  results.  Castle  did  not  at  first  admit  the  ade- 
quacy of  this  explanation,  but  continued  to  maintain  that  he  had 
modified  a  unit  character  by  selection,  and  some  geneticists  agreed 
with  him. 

This  question  might  be  raised.  Why  cling  so  strongly  to  the 
cumulative  factor  hypothesis  and  force  Castle's  results  into  this 
conception  ?  Is  there  anything  sacred  about  a  unit  character  that 
it  should  not  be  modified  just  as  complex  chemical  molecules  may 
be  modified  in  certain  reactions'?  Why  not  admit  that  Mendelian 
factors  may  be  modified,  and  explain  Castle's  results  in  this  way  ? 
The  reason  is  that  when  we  begin  to  admit  that  unit  characters 
and  single  MendeUan  factors  may  be  modified,  the  whole  con- 
ception of  inheritance  becomes  chaos.  The  great  advantage  of 
the  factor  hypothesis  is  that  it  furnishes  the  clearest  method  of 
describing  breeding  results.  East  (4)  makes  an  eloquent  state- 
ment on  this  point. 

*' Taking  into  consideration  all  the  facts,  no  one  can  well  deny 
that  they  are  well  described  by  terminology  which  requires  hypo- 
thetical segregating  units,  as  represented  by  the  term  'factor.' 
What  then  is  the  object  of  having  the  units  vary  at  will  ?  There 
is  then  no  value  to  the  unit,  the  unit  itself  being  only  an  assumption. 
It  is  the  expressed  character  that  is  seen  to  vary;  and  if  one  can 
describe  these  facts  by  the  use  of  hypothetical  units,  theoretically 
fixed,  but  influenced  by  the  environment  and  by  other  units,  sim- 
plicity of  description  is  gained.  If,  however,  one  creates  a  hypo- 
thetical unit  by  which  to  describe  phenomena,  and  this  unit  varies, 
he  really  has  no  basis  for  description." 

The  question  was  finally  settled  in  a  very  neat  way  by  some 
critical  breeding  experiments  that  Castle  himself  performed  on 
these  same  hooded  rats  (2).  The  degree  of  pigmentation  on  the 
high  pigmentation  strain  was  designatetl  as  +3.73  (in  terms  of 


Q4  Outline  of  Genetics 

certain  arbitrary  units),  while  the  low  pigmentation  strain  was 
-2.63.  Critical  examination  revealed  the  fact  that  the  cross 
between  the  +3.73  strain  and  the  wild  race  brought  a  slight  reduc- 
tion in  the  amount  of  pigmentation  as  it  appeared  in  the  extracted 
hooded  "recessives"  in  the  F2.  Repeated  recrossing  of  these 
extracted  individuals  with  the  wild  race  finally  resulted  in  extracted 
hooded  rats  of  the  grade  +3-04.  No  further  reduction  was  pos- 
sible in  this  way.  These  results  could  be  explained  by  the  follow- 
ing assumptions.  The  hooded  pattern  is  modified  in  degree  of 
pigmentation  by  a  varying  number  of  doses  of  cumulative  factors 
(as  the  ''mutationists"  had  previously  maintained).  The  wild 
race  is  characterized  by  having  a  certain  number  of  doses  of  these 
cumulative  factors.  The  repeated  crossings  and  extractions 
mentioned  above  would  eventually  result  in  producing  rats  which 
had  the  hooded  pattern  plus  that  number  of  doses  of  cumulative 
factors  which  was  characteristic  of  the  germ  plasm  of  the  wild 
race.  Since  it  was  found  that  repeated  crosses  with  the  wild  race 
could  bring  the  degree  of  pigmentation  down  to  +3-04  and  no 
lower,  it  was  felt  that  +3.04  was  the  degree  of  pigmentation  which 
would  be  produced  by  that  number  of  cumulative  factors  which 
was  characteristic  of  the  wild  race. 

The  critical  test  of  these  assumptions  could  be  made  through 
a  similar  manipulation  of  the  low  pigmentation  strain.  If  the 
assumptions  were  correct,  the  low  pigmentation  strain  should  be 
raised  finally  to  +3-04  by  repeated  crossing  with  the  wild  race. 
Castle  performed  this  experiment  and  got  exactly  this  result, 
one  of  the  families  from  the  low  pigmentation  strain  (-2.63)  being 
finally  brought  up  to  -I-3.05. 

These  results  naturally  caused  Castle  to  change  his  views 
on  the  matter,  and  served  rather  generally  to  establish  the  views 
of  the  ''mutationists."  The  situation  depended  for  its  interpre- 
tation upon  the  cumulative  factor  mechanism.  (Here  it  was  felt 
that  the  cumulative  factors  were  not  primarily  responsible  for 
the  production  of  the  character  in  question,  but  served  merely 
to  modify  the  degree  in  which  it  expressed  itself.  Other  cases  of 
the  same  sort  have  been  encountered  elsewhere,  the  mechanism 
at  play  being  commonly  referred  to  in  the  literature  as  "multiple 
modifying  factors.") 


InJieritance  of  Quantitative  Characters  95 

LITERATURE  CITED 

1.  Castle,  W.  E.,  The  inconstancy  of  unit  characters.  Amcr. 
Nat.  46:352-362.  1912. 

2.  ,  Piebald  rats  and  the  theory  of  genes.     Proc.  Nat. 

Acad.  Sci.  5 : 1 26-130.  jig.  i.  1919. 

3.  De  Vries,  H.,  Species  and  varieties,  their  origin  by  mutation. 
Chicago.  1905. 

4.  East,  E.  M.,  The  Mendehan  notation  as  descriptive  of  physio- 
logical facts.     Amer.  Nat.  46:633-655.  191 2. 

5.  Emerson,  R.  A.,  and  East,  E.  M.,  The  inheritance  of  quantita- 
tive characters  in  maize.  Bull.  Agric.  Exper.  Sta.  Nebr.  no.  2 
pp.  120.  figs.  21.  1913. 

6.  Nilsson-Ehle,  H.,  Einige  Ergebnisse  von  Kruzungen  bei 
Hofer  und  Weizen.     Bot.  Notiser  1908:257-294. 

7.  ,    Kreuzungsuntersuchungcn    an    Hafer    und    Weizen. 

Lands.  Univ.  Arsskr.  N.S.  II.  5:1-122.  1909. 


CHAPTER  VI 
LINKAGE 

The  fundamental  mechanism  of  inheritance  which 
was  proposed  by  Mendel,  and  which  was  later  supported 
by  cytological  studies,  has  been  confirmed  time  and 
again  by  breeding  experiments.  Its  scope  is  consid- 
erably enlarged  by  the  factor  hypothesis,  but  its  basic 
concepts  are  not  altered.  It  will  now  be  necessary  to 
consider  some  well-established  facts  of  inheritance  which 
can  be  interpreted  only  by  analyzing  still  further  the 
hereditary  mechanism. 

It  has  been  assumed  that  the  chromosomes  are  the 
bearers  of  the  hereditary  units  or  genes.  (The  term 
''gene"  is  used  where  it  is  not  intended  to  imply  whether 
the  hereditary  unit  acts  as  a  factor  or  determiner.)  This 
has  been  warranted  by  the  fact  that  the  distribution  of 
the  chromosomes  in  inheritance  fits  exactly  into  the  Men- 
delian  scheme.  In  the  cases  that  have  been  considered 
in  the  last  few  chapters,  the  genes  have  always  been 
located  on  separate  chromosomes,  with  the  result  that 
they  have  been  passed  on  in  inheritance  quite  independ- 
ently of  each  other.  The  intensive  study  of  inheritance 
that  has  been  made  during  the  last  decade,  however,  has 
revealed  cases  where  the  total  number  of  genes  known  for 
the  organism  exceeds  the  number  of  chromosome  pairs. 
The  obvious  conclusion  is  that  more  than  one  gene  may 
be  carried  on  a  single  chromosome.  If  this  is  true,  it 
should  result  in  decided  modifications  of  the  breeding 

96 


Linkage  97 

results.  Conversely,  the  occurrence  of  a  certain  type  of 
breeding  result  would  serve  as  a  clear  indication  that 
more  than  one  gene  may  be  carried  on  a  single  chromo- 
some, and  thus  as  a  further  confirmation  of  the  belief 
that  the  chromosomes  are  the  bearers  of  hereditary 
characters. 

In  191 1,  results  of  just  this  sort  were  obtained  in 
corn  by  Emerson  (2),  who  stated:  ''This  is  an  example 
of  a  feature  w^hich  is  probably  very  widespread  in  the 
plant  world,  but  of  which  at  present  we  know  little." 
Long  before  any  further  important  work  was  done  along 
this  line  among  plants,  however,  Morgan  (5)  published 
the  results  of  his  very  careful  and  intensive  breeding 
experiments  with  the  fruit  fly.  His  ideas  have  had  a 
profound  influence  upon  subsequent  work  in  genetics. 
He  has  given  us  a  more  accurate  picture  of  the  hereditary 
mechanism  and  one  that  fits  the  facts  bettei  than  any 
previously  proposed.  In  simplest  terms  the  picture  is 
this.  Each  chromosome  is  a  rodlike  structure,  and 
numerous  genes  are  arranged  in  a  line  along  this  rod. 
Thus  Morgan  further  analyzes  the  germ  plasm  by  accu- 
rately locating  the  genes.  (He  does  not  attempt  any 
description  in  physico-chemical  terms  of  the  genes  them- 
selves or  of  the  exact  relation  they  may  hold  to  the 
chromosomes  on  which  they  are  carried.) 

We  cannot  discuss  here  the  many  ways  in  which  this 
fundamental  conception  has  cast  light  upon  work  in 
genetics.  Suffice  it  to  say  that  it  has  resulted  in  a  new 
''school"  of  geneticists  whose  experiments  have  been 
more  intensive,  more  exact,  and  in  s(^mc  ways  more 
"fundamental"  than  those  of  any  previous  school.  To 
date  most  of  the  linkage  work  has  been  done  with  the 


98 


Outline  of  Genetics 


fruit  fly,  but  a  great  deal  of  information  is  now  being 
accumulated  on  linkage  in  corn,  and  numerous  scattered 
demonstrations  of  the  phenomenon  have  been  made  in 
other  organisms  as  well.  Only  a  rather  simple  explana- 
tion will  be  attempted  here,  to  bring  out  merely  some  of 
the  fundamental  principles  of  the  phenomenon. 

When  first  considering  Mendel's  law,  the  state- 
ment was  made  that  more  than  one  determiner  might 
be  located  on  a  given  chromosome.     As  yet  we  have 


© 

© 

© 
© 

Tall  Red  Parent 

© 
© 

© 

© 

Dwarf  White  Parent 


4  Possible  Gametes 


.-.  F,  Shows  9:3:3.1  Ratio 


Fig.   1 6. — Diagram  showing  normal  dihybrid  behavior  when  no 
linkage  is  involved. 

considered  no  such  case,  but  linkage  involves  exactly 
this  situation.  In  connection  with  some  of  Mendel's 
original  crosses  fig.  i6  will  be  recalled.  In  this  case  a 
double  dominant  mates  with  a  double  recessive,  and  the 
result  is  a  dihybrid  ratio  in  the  F2  generation,  following 
the  production  of  four  types  of  gametes  by  the  Fi  plants. 
Suppose,  however,  that  the  determiner  T  and  the  deter- 
miner R  are  carried  on  the  same  chromosome,  the  situa- 
tion would  be  as  represented  in  fig.  17.  Here  the  Fj 
individuals  produce  only  two  types  of  gametes,  as  in  a 
monohybrid,  so  that  the  F2  presents  what  amounts  to 


Linkage 


99 


a  monohybrid  ratio,  the  tall  individuals  always  being 
fed  flowered,  and  the  dwarf  individuals  always  white 
flowered.  The  results  obviously  arise  from  the  fact 
that  T  and  R  are  linked,  being  located  on  the  same  chro- 
mosome, as  are  also  /  and  r. 

These  linkage  results,  taken  by  themselves,  might 
seem  to  recommend  the  following  interpretation.  It  is 
really  the  chromosome  itself  that  is  the  important  and 
indivisible    unit    in   inheritance,    while    the    distinction 


"LINKAGE" 


Dwarf  White  Parent 


Only  2  Gametes 

Possible 
.'.  F2  Shows  3  :  i  Ratio 


Fig.  17. — Diagram  showing  "dihybrid"  behavior  when  genes  are 
linked.     Fi  produces  only  two  types  of  gametes. 


between  the  various  genes  on  a  single  chromosome  is 
purely  arbitrary  and  unnecessary.  In  other  words,  why 
need  we  assume  that  T  and  R  are  distinct  genes,  when  we 
would  be  equally  justified  in  assuming  that  tallness  and 
redness  are  merely  two  of  the  effects  })roduced  by  the 
same  chromosome  ?  This  latter  assumption  may  appear 
attractive,  but  it  becomes  clearly  im])ossil)lc  when  some 
of  the  further  breeding  results  arc  considered. 

Following  out  the  foregoing  example  (merely  as  an 
illustration),  when  it  was  discovered  that  tall  individuals 
always  had  red  flowers,  this  fact  was  explained  as  linkage. 


loo  Outline  of  Genetics 

The  inference  was  that  in  these  same  cultures  there  could 
never  appear  a  tall  plant  with  white  flowers  nor  a  dwarf 
plant  with  red  flowers,  for  if  there  were  linkage,  and  the 
chromosome  were  the  indivisible  unit  in  inheritance,  it 
would  be  impossible  for  tallness  and  redness  to  become 
separated.  As  a  matter  of  fact,  it  was  soon  recognized 
that  these  'impossible"  individuals  did  actually  occur. 
Small  numbers  of  tall  whites  and  dwarf  reds  regularly 
appeared  among  the  same  cultures  in  which  the  linkage 
of  tallness  and  redness  had  been  demonstrated,  and  the 
work  had  been  done  under  such  conditions  of  control 
that  there  could  have  been  no  experimental  error. 

This  new  fact  demanded  an  explanation,  for  with 
such  chromosomes  as  TR  and  tr  it  would  be  impossible  to 
obtain  a  tall  white  individual  so  long  as  the  individuality 
of  the  chromosome  was  maintained.  When  chromosomes 
were  examined  with  the  modern  lenses  they  were  found 
to  show  all  kinds  of  tangled  contortions  during  the 
reduction  division,  and  accordingly  the  scheme  shown 
in  fig.  1 8  was  devised.  These  five  stages  represent  phases 
that  an  allelomorphic  pair  of  chromosomes  may  go 
through  during  reduction  division.  This  pair  of  chromo- 
somes, which  would  normally  lie  side  by  side  (i),  may 
at  times  come  to  lie  across  one  another  (2).  In  this 
position  the  middle  regions  of  the  chromosomes  are  in 
contact  and  are  conceived  of  as  fusing  (3) .  The  spindle 
fibers  from  each  pole  then  lay  hold  of  this  compound 
chromosome  and  the  pull  comes  in  the  direction  of  the 
arrows  shown  in  the  figure.  This  results  in  the  break 
indicated  in  (4).  Finally,  two  new  chromosomes  separate 
from  the  old  compound  chromosome,  as  indicated  in  (5). 
Thus  T  becomes  linked  with  r,  and  later,  when  a  mating 


Linkage 


lOI 


occurs  between   two  gametes,  each   of  which   contains 
such  a  chromosome,  the  result  is  a  tall,  white-llowered 


R 


T 


2 


3 


t      ) 


R 


R 


Fig.  iS. — Illustrating  how  crossing  over  may  occur 


I02  Outline  of  Genetics 

individual.  In  the  same  way  and  with  equal  likelihood, 
dwarf,  red-flowered  individuals  may  appear. 

This  scheme  serves  to  account  for  the  occurrence  of 
the  ^'exceptional"  individuals  in  linkage  cultures.  The 
whole  phenomenon  is  known  as  crossing  over.  It  has 
been  practically  impossible  to  provide  a  direct  demon- 
stration that  the  chromosomes  behave  in  exactly  this 
manner  during  the  reduction  division,  but  there  has 
accumulated  an  enormous  mass  of  indirect  evidence  from 
the  breeding  results  to  support  this  view.  Evidently 
the  chromosome  is  not  the  indivisible  unit  in  inheritance, 
but  is  divisible  according  to  a  rather  regular  scheme. 
Whole  sections  may  be  evenly  exchanged  between  the 
members  of  an  allelomorphic  pair  of  chromosomes. 

Once  the  phenomenon  of  crossing  over  had  been 
identified,  investigation  was  undertaken  to  determine 
the  regularity  and  frequency  of  the  phenomenon.  It 
was  discovered  that  the  amount  of  crossing  over  that 
took  place  between  a  given  pair  of  genes  had  a  constant 
value.  For  example,  lo  per  cent  of  the  crossing  over 
could  be  depended  on  to  occur  between  T  and  R  in  every 
experiment  involving  these  two  determiners.  The 
exact  cross-over  value  is  of  course  computed  from  the 
breeding  results  obtained.  In  the  present  example,  a 
cross-over  value  of  lo  per  cent  between  T  and  R  would 
work  out  as  follows.  In  the  reduction  division  (in  the 
Fi  hybrid  which  results  from  tall  red  X dwarf  white), 
crossing  over  takes  place  in  lo  per  cent  of  the  cases, 
while  crossing  over  fails  and  the  original  linkage  relation- 
ships are  maintained  in  90  per  cent  of  the  cases.  As  a 
result,  four  types  of  gametes  are  produced  in  the  follow- 
ing frequencies:  45  per  cent  TR,  45  per  cent  /r,  5  per 


Linkage 


103 


cent  Tr;  5  per  cent  tR;  or  9  TR,  9  Ir,  i  7>,  i  IR.  'J1ie  F^ 
population  which  results  from  the  random  matings  among 
this  assortment  of  gametes  is  represented  in  fig.  19,  the 
phenotypic  ratio   being    281    tall  red:  19   tall  white:  19 


R       R 


81 


^T)  Cl 


R 


fT\ 


R 


rT\ 


R 


R 


R 


T 


R 


81 


vly 


^f  t 


R  Ji^ 
9  9 


^^T7^ 


R 


R      R 


R 


II 


12 


R 


t 


R 


t       t 


tut 


13 


14 


16 


•  Fig.  19. — Showing  F2  population  produced  by  random  mating  of 
gametes  of  Fi  in  a  case  of  linkage  with  10  per  cent  crossing  over. 

dwarf  red: 81  dwarf  w^hite.  In  such  cases,  of  course,  the 
original  investigator  has  for  his  data  only  this  final  pheno- 
typic ratio,  and  from  these  data  must  compute  the 
amount  of  crossing  over  that  has  taken  place.  In  actual 
practice  this  computation  would  be  simplified  by  the 
use  of  a  formula.     As  a  matter  of  fact,  the  necessity  of 


I04 


Outline  of  Genetics 


using  any  such  formula  can  usually  be  avoided  through 
the  following  expedient.  Instead  of  inbreeding  the  Fi, 
it  can  be  back  crossed  with  the  double  recessive  parent 
(dwarf  white).  This  parent  race  is  perfectly  homozy- 
gous, so  that  it  produces  only  the  one  type  of  gamete 
(in  spite  of  crossing  over,  which  must  be  taking  place 
here  also).  The  results  of  this  back  cross  are  represented 
in  fig.  20.  It  is  obvious  that  the  phenotypic  ratio 
obtained  (9  tall  red:i  tall  white:  i  dwarf  red:  9  dwarf 


T 
R 


R 


Fig.  20 — Population  resulting  from  mating  of  gametes  of  Fi  (shown 
above)  with  gametes  of  recessive  parent  (only  one  type  of  gamete,  shown 
at  left)  in  a  case  of  linkage  with  lo  per  cent  crossing  over. 

white)  corresponds  exactly  to  the  ratio  among  the  types 
of  gametes  produced  by  the  Fj.  In  this  way  the  cross- 
over value  is  quite  apparent,  and  no  computation  neces- 
sary. Back  crosses  wdth  the  recessive  parent  will  always 
provide  results  which  are  easier  to  interpret  than  are  the 
F2  ratios. 

As  investigations  were  made  of  additional  pairs  of 
linked  genes,  it  was  discovered  that  each  such  pair  had 
a  characteristic  and  rather  constant  cross-over  value. 
For  example,  assuming  that  a  third  gene  A  is  located  on 
the  same  chromosome  w^ith  T  and  Ry  by  means  indicated 


Linkage  105 

above  a  test  is  made  as  to  the  cross-over  \'alue  between 
A  and  T.  This  value  is  discovered  to  be  20  per  cent. 
With  these  two  cross-over  values  determined,  consider- 
able interest  now  becomes  focused  upon  the  relations  of 
A  and  R.  Suitable  tests  reveal  the  fact  that  the  cross- 
over value  between  A  and  R  is  30  per  cent.  When  three 
such  cross-over  values  as  these  are  considered  together, 
Morgan's  theoretical  scheme  of  the  'linear  arrange- 
ment of  genes"  is  immediately  suggested,  for  these 
results  can  best  be  interpreted  on  the  following  basis. 
The  genes  are  arranged  in  a  line  on  the  chromosome, 
and  the  frequency  of  crossing  over  between  any  two 
genes  depends  upon  their  distance  apart.  Using  arbi- 
trary units  to  correspond  with  the  actual  percentage 
of  crossing  over,  the  three  genes  employed  in  the  example 
may  be  arranged  in  the  order  ATR,  with  A  and  T  20 
units  apart,  T  and  R  10  units  apart,  and  A  and  R,  there- 
fore, 30  units  apart.  (The  chromosome  map  first  de- 
vised, e.g.,  fig.  18,  would  then  have  to  be  modified  some- 
what to  fit  these  new  facts.) 

It  is  in  this  manner  that  Morgan  and  his  students 
have  been  able  to  construct  rather  startling  chromo- 
some maps,  indicating  in  a  very  exact  way  the  relative 
position  and  spacing  of  scores  of  genes  on  a  single  chro- 
mosome. Striking  confirmation  of  the  scheme  appears 
in  the  following  fact.  The  fruit  fly  possesses  four  chro- 
mosome pairs,  one  of  which  is  visibly  much  shorter  than 
the  others.  Breeding  results  reveal  that  the  many  known 
genes  are  associated  in  just  four  ''linkage  groups,"  one 
of  the  four  groups  containing  a  much  smaller  number 
of  genes  than  the  other  three,  and  being  distinctly 
''shorter,"  as  mapped  from  the  cross-over  values.     In 


io6  Outline  of  Genetics 

truth,  this  scheme  of  Morgan's  has  such  an  enormous 
mass  of  data  to  support  it  that,  for  all  practical  purposes, 
it  may  be  regarded  as  an  established  fact. 

Cross-over  values  in  the  fruit  fly  may  run  as  high  as  80  per 
cent,  and  in  one  of  the  related  species  much  higher  values  have 
been  reported  (Lancefield  4).  Such  a  high  cross-over  value 
seems  rather  surprising,  for  it  represents  a  case  where  the  cross- 
overs are  much  more  frequent  than  the  non-cross-overs.  It 
should  be  reahzed  that  a  cross-over  value  of  50  per  cent,  where 
cross-overs  and  non-cross-overs  are  equally  frequent,  would  give 
exactly  the  same  breeding  results  as  if  the  two  genes  in  question 
were  located  on  different  chromosome  pairs.  Similarly,  cross- 
over values  higher  than  50  per  cent  would  give  the  same  breeding 
results  as  though  the  linkage  were  reversed;  that  is,  if  M  and  N 
were  80  units  apart,  the  breeding  results  of  a  single  experiment 
involving  M  and  N  only  would  seem  to  indicate  that  M  was  linked 
with  n  and  m  with  N.  It  is  evident,  therefore,  that  these  higher 
cross-over  values  are  computed  from  a  considerable  set  of  ex- 
periments. Every  newly  discovered  gene  is  carefully  tested  with 
at  least  two  other  genes  whose  position  is  already  known,  and 
thus  the  new  gene  is  accurately  placed  on  the  chromosome. 

The  cases  that  have  just  been  discussed  are  known  as  single 
cross-overs;  the  two  chromosomes  of  the  pair  come  to  lie  across 
one  another  at  a  single  point,  and  a  single  break  with  the  subse- 
quent rearrangement  is  sufficient  to  account  for  the  results.  In 
view  of  the  physical  mechanism  which  seems  to  be  responsible 
for  these  cross-overs,  it  is  not  surprising  to  find  that  there  may 
sometimes  occur  double  cross-overs.  In  these  cases  the  two 
chromosomes  come  to  lie  across  one  another  at  two  points,  and  a 
break  takes  place  at  each  point,  with  the  corresponding  exchange 
of  chromosome  regions.  This  amounts  to  an  even  exchange  of 
corresponding  zones  from  the  middles  of  the  chromosomes,  with 
the  two  end  zones  on  each  chromosome  remaining  as  before.  Inas- 
much as  crossing  over  of  any  sort  is  detected  only  through  its 
effect  on  the  breeding  results,  double  crossing  over  can  be  demon- 
strated only  in  experiments  that  involve  observations  on  at  least 
three  genes  that  are  rather  widely  separated  on  the  same  chromo- 


Linkage  107 

some.  As  would  be  expected,  double  cross-overs  are  inucli  less 
frequent  than  single  cross-overs.  There  have  also  been  reported 
very  rare  cases  of  triple  cross-overs. 

The  question  arises  whether  the  frequency  of  crossing  over 
depends  strictly  and  only  upon  the  real  distances  between  the 
genes  on  a  chromosome.  In  the  first  place,  it  is  theoretically 
possible  that  certain  regions  of  a  chromosome  may,  owing  per- 
haps to  physical  peculiarities,  lend  themselves  more  readily  to 
crossing  over  than  do  other  regions  of  the  same  chromosome.  (A 
suggestion  to  this  effect  appears  in  some  of  Muller's  work,  see 
chapter  on  ''Mutation. ")  This  possibility  is,  of  necessity,  ignored 
in  the  chromosome  maps,  which  are  constructed  purely  on  the 
practical  assumption  that  crossing  over  is  proportional  to  distance. 
In  the  second  place,  it  is  recognized  that  certain  special  influences, 
such  as  temperature  and  age  of  the  organism,  may  modify  the 
normal  frequency  of  crossing  over.  Dependable  values  on  crossing 
over  are  to  be  obtained,  therefore,  only  when  such  conditions  are 
absolutely  standardized  (Sturtevant  6).  Further,  one  of  the 
outstanding  and  unexplained  peculiarities  of  this  phenomenon  is 
that,  in  the  fruit  fly,  crossing  over  takes  place  freely  in  the  female, 
while  none  whatsoever  takes  place  in  any  of  the  chromosome  pairs 
of  the  male.  It  is  interesting  to  note  that  in  organisms  where  the 
female  is  heterozygous  for  sex — female  homozygous  for  sex  in  the 
fruit  fly — exactly  the  reverse  is  true,  crossing  over  taking  place  in 
the  male  but  not  in  the  female  (Tanaka  7).  In  corn,  where  both 
sexes  are  represented  on  the  same  individual,  there  is  no  such 
restriction,  for  crossing  over  takes  place  with  approximately 
equal  frequency  in  microsporogenesis  as  in  megasporogenesis 
(Emerson  and  Hutchinson  3).  Finally,  it  has  been  demon- 
strated that  the  presence  of  certain  special  genes  will  modify  the 
frequency  of  crossing  over,  so  that  it  has  been  possible  to  isolate 
races  of  the  fruit  fly  in  which  an  extremely  high  or  an  extremely 
low  rate  of  crossing  over  takes  place  (Sturtevant  6,  Detlef- 

SEN  l). 


io8  Outline  of  Genetics 

LITERATURE  CITED 

1.  Detlefsen,  J.  A.,  Is  crossing  over  a  function  of  distance? 
Proc.  Nat.  Acad.  Sci.  6:663-670.  1920. 

2.  Emerson,  R.  A.,  Genetic  correlation  and  spurious  allelomor- 
phism in  maize.  Connecticut  Agric.  Exp.  Sta.  Bull.  167:1-142. 
1911. 

3.  Emerson,  R.  A.,  and  Hutchinson,  C.  B.,  The  relative  fre- 
quency of  crossing  over  in  microspore  and  megaspore  develop- 
ment in  maize.     Genetics  6:417-432.  1921. 

4.  Lancefield,  D.  E.,  Linkage  relations  of  the  sex-linked  char- 
acters in  Drosophila  obscura.     Genetics  7:335-384.  1922. 

5.  Morgan,  T.  H.,  Heredity  and  sex.     New  York.  1914. 

6.  Sturtevant,  a.  H.,  Contributions  to  the  genetics  of  Droso- 
phila melanogaster.  HI.  Inherited  linkage  variations  in  the 
second  chromosome.  Carnegie  Inst.  Wash,  Publ.  278 :  305-341 . 
1919. 

7.  Tanaka,  Yoshimaro,  Genetic  studies  on  the  silkworm.  Jour. 
Coll.  Agric.     Sapporo  7:129-255.  ph.  4.  1916. 


CHAPTER  VII 
MUTATION 

When  De  Vries  ''discovered"  the  phenomenon  of 
mutation  in  Oenothera  Lamarckiana,  he  stated  that 
mutations  were  quahtative,  discontinuous,  constant 
changes  in  the  germ  plasm  (see  p.  6).  These  three 
fundamental  characteristics  still  hold  true,  but  some  of 
De  Vries'  other  ideas  have  been  considerably  qualilied 
by  later  work.  The  critical  analysis  of  the  germ  plasm 
that  has  been  effected  during  the  last  decade  has  made 
it  possible  to  describe  mutation  in  a  much  more  exact 
way  than  before,  and  to  describe  it  in  terms  of  the 
Mendelian  mechanism. 

For  convenience  our  discussion  of  this  general  subject 
will  be  put  into  the  form  of  a  classification.  (The 
sequence  followed  in  this  classihcation  is  that  of  the 
increasing  magnitude  of  the  "area"  of  the  germ  plasm 
affected  by  the  change.) 

I.     gene  changes 

I.  Locus  changes. — These  are  changes  restricted 
to  a  single  locus  of  one  of  the  chromosomes,  so  that  they 
involve  only  one  gene,  without  affecting  even  its  nearby 
neighbors.  Usually  they  are  effective  on  only  one  chro- 
mosome of  a  pair,  without  affecting  the  corresi)onding 
locus  of  its  allelomorpliic  mate.  Consequently,  the 
change  first  appears  in  the  heterozygous  condition. 
(Baur  estimates  that  such  changes  originate  in  the 
heterozygous  condition  four  hundred  times  as  frecjuently 

109 


no  Outline  of  Genetics 

as  in  the  homozygous.)  They  are  mostly  "loss"  mutations 
and  recessive  to  the  previous  condition.  Only  a  very  few 
dominant  or  "gain"  mutations  have  ever  been  reported. 

Baur  (i),  working  with  Antirrhinum,  concludes  that 
changes  of  this  sort  take  place  more  frequently  in  the 
vegetative  tissues  than  in  connection  with  gametogenesis . 
The  earlier  work  on  the  fruit  fly  indicated  that  the 
locus  changes  took  place  late  in  gametogenesis,  since  only 
one  individual  of  the  new  type  would  appear  in  a  pro- 
geny. Later  investigation,  however,  has  indicated  that 
the  change  may  take  place  at  almost  any  point  in  onto- 
geny (Bridges  4,  Muller  8).  (There  are  also  indica- 
tions that  changes  of  this  sort  may  take  place  in  purely 
somatic  tissue,  although  in  such  cases,  of  course,  the  modi- 
fication cannot  be  perpetuated.  See  also  chapter  viii 
on  this  matter.) 

Zeleny  (12)  states  that  there  is  no  periodicity  to 
these  mutations,  thus  refuting  one  of  the  early  ideas  of 
De  Vries.  The  same  investigator  demonstrates  that 
reverse  mutations  are  more  frequent  than  original  muta- 
tions. (This,  however,  is  simply  because  they  are  in 
the  reverse  direction,  and  not  because  of  their  recent 
origin.)  In  the  case  of  these  reverse  mutations,  the 
changes  are  always  full  jumps  back  to  the  original 
starting-point,  and  never  result  in  an  intermediate  con- 
dition; nor  will  the  selection  of  extreme  types  at  all 
modify  the  rate  at  which  these  reverse  mutations  occur. 

MuLLER  and  Altenburg  (10),  who  have  conducted 
a  critical  examination  of  the  fruit  fly  for  mutations 
occurring  on  the  first  and  second  chromosomes,  state 
that  the  vast  majority  of  locus  changes  have  a  lethal 
or  semilethal  effect  when  present  in  the  homozygous 


Mutation 


III 


(recessive)  condition.  (It  is  obvious,  therefore,  that  a 
critical  search  for  these  mutations  must  in\'olve  a  very 
special  technique.  These  authors  are  in  possession  of 
such  a  technique  through  their  intimate  knowledge  of 
the  linkage  groups  on  the  chromosomes  in  question,  and 
their  ability  to  detect  the  absence  of  certain  expected 
classes.)  On  one  chromosome  they  uncovered  the  start- 
ling fact  that  50  per  cent  of  the  mutations  were  located 
in  a  restricted  region  at  one  end  of  the  chromosome,  which 
amounted  to  about  2  per  cent  of  its  length  as  charted 
from  cross-over  values.  (It  is  an  open  question  whether 
this  indicates  a  highly  mutable  region  of  the  chromosome, 
or  whether  cross-over  values  are  an  inaccurate  index  of 
length.) 

The  most  promising  phase  of  Muller's  work  arises 
from  his  critical  study  of  the  rate  of  mutation.  Consider- 
ing the  whole  length  of  the  first  chromosome  of  the  fruit 
fly,  one  mutation  occurs  in  106  gametes.  For  the  second 
chromosome  the  corresponding  value  is  one  in  175 
gametes.  Zeleny  states  that  locus  changes  occur  as 
frequently  in  one  sex  as  in  the  other.  Having  estab- 
lished these  constants,  Muller  is  now  investigating  the 
possibility  of  modifying  the  normal  rate  of  mutation. 
Already  he  has  been  successful  in  depressing  the  rate  one- 
half  by  means  of  low  temperatures.  Eventually  such 
knowledge  may  be  turned  to  some  practical  \'alue. 

Two  further  points  should  be  mentioned  about  the 
locus  changes.  Variation  of  this  type  has  been  encoun- 
tered (or  at  least  identified)  much  more  frequently  than 
have  any  of  the  other  types  of  changes  mentioned  below. 
The  term  "mutation"  is  usually  restricted  by  geneti- 
cists to  apply  to  locus  changes. 


112  Outline  of  Genetics 

The  presence  and  absence  hypothesis  has  been  very  generally 
accepted  because  of  its  value  in  simpHfying  our  concepts  and  pro- 
viding the  most  convenient  scheme  of  notation.  At  the  same  time 
it  is  recognized  that  this  hypothesis  may  not  strictly  represent  the 
truth  in  all  cases.  There  are  two  outstanding  types  of  evidence 
that  can  hardly  be  interpreted  by  means  of  the  presence  and 
absence  concept.  One  of  these  will  be  discussed  later  in  another 
connection  (p.  151).     The  other  is  as  follows. 

In  some  cases  other  possibihties  may  be  realized  in  connection 
with  a  single  locus  than  merely  the  presence  or  absence  of  a  given 
gene.  For  example,  at  a  given  locus  on  one  of  the  chromosomes  in 
corn,  a  condition  W  may  exist,  which  results  in  colorless  pericarp. 
In  other  plants  that  same  locus  may  bear  the  gene  F,  variegated 
pericarp,  and  in  still  others  S,  completely  colored  pericarp.  S  is 
dominant  to  V  and  V  to  W  (see  also  p.  119).  As  a  matter  of  fact, 
these  three  are  simply  representatives  of  a  series  of  ten  different 
conditions  that  may  be  present  at  a  given  locus.  Such  cases  are 
spoken  of  as  systems  or  series  of  multiple  allelomorphs,  and  it  would 
be  difficult  to  harmonize  them  strictly  with  the  presence  and 
absence  hypothesis. 

The  relation  of  systems  of  multiple  allelomorphs  to  mu- 
tation is  significant.  It  would  be  possible  to  arrange  the  genes 
involved  in  any  system  of  multiple  allelomorphs  in  a  series, 
placing  at  the  top  the  one  which  was  dominant  to  all  the 
others,  and  at  the  bottom  the  one  that  was  recessive  to  all  the 
others.  This  series,  however,  in  no  way  reflects  the  order  in  which 
such  genes  have  originated  by  mutation.  In  the  fruit  fly,  there 
is  a  famous  series  of  multiple  allelomorphs  for  eye  color,  ranging 
from  white  through  progressive  steps  in  intensity  of  coloration; 
but  it  is  not  true  that  white  first  mutated  to  a  light  shade,  which 
later  mutated  to  the  next  darker  shade,  and  so  on;  nor  is  it  true 
that  this  series  of  mutants  came  off  in  a  regular  sequence  down  the 
scale  of  color  intensity.  In  short,  within  a  series  of  multiple 
allelomorphs  the  mutants  come  off  in  discontinuous  rather  than 
continuous  series.  It  follows  that  mutations  cannot  be  "led 
along"  in  a  given  direction  by  means  of  selection. 

Further,  although  it  is  common  that  all  the  genes  in  a  series  of 
multiple  allelomorphs  affect  the  same  general  character,  exceptions 


Mutation  113 

to  this  have  been  noted.  Muller  (9)  cites  a  case  in  which  the 
different  mutant  genes  at  the  same  locus  may  cause  either  shorten- 
ing of  wing,  eruption  on  thorax,  or  a  lethal  effect. 

At  the  same  time,  although  we  are  thus  repeatedly  encounter- 
ing evidence  on  the  discontinuity  of  mutation,  it  is  possible  that 
there  is  an  underlying  continuity  of  a  sort  that  we  are  not  in  a 
position  to  measure.  A  statement  of  JVIorgan's  (6)  bears  on  this 
point.  "Evidence  is  fast  accumulating  that  common  genes 
probably  undergo  analogous  mutation  in  related  species,  the 
direction  being  conditioned  by  the  physico-chemical  constitution 
of  the  gene  and  not  by  some  hypothetical  directive  force." 

2.  CoMPLEXMUTATiONS. — It  is  perhaps  surprising  that,  in 
spite  of  the  numerous  cases  of  locus  changes  that  were  being 
discovered,  there  were  for  a  long  time  no  clear  cases  of  mutations 
involving  simultaneous  changes  in  several  neighboring  factors  in 
one  region  of  a  chromosome.  Nilsson-Ehle  (ii)  now  claims  to 
have  such  a  case,  and  calls  it  ''complexmutation."  Normal  wheat 
mutates  to  bearded  speltoid,  involving  a  simultaneous  change 
in  two  closely  linked  genes.  Among  the  F2  progeny  of  normal  A' 
mutant  appear  a  few  bearded  normal  type  and  beardless  speltoid, 
but  only  a  few,  due  to  the  very  close  linkage  of  the  two  mutated 
genes.  In  another  case  the  same  investigator  claims  that  three 
linked  factors  have  mutated  simultaneously. 

3.  Deficiency. — A  rare  phenomenon  has  been  described  by 
Bridges  (3),  working  on  the  fruit  fly.  "Deficiency"  as  he  calls 
it,  is  something  more  extensive  than  a  simple  locus  change  (and 
probably  more  extensive  than  the  complexmutations).  It  is  a 
"regional  mutation,"  involving  an  "inactivation"  of  a  portion  of 
a  chromosome,  so  that  the  genes  on  that  region  of  the  chromosome 
are  rendered  ineffective  (nor  can  crossing  over  take  place  in  that 
region). 

ii.     chromosome  changes  ('' chromosome 
aberrations") 

I.  Duplication.— Bridges  (3)  describes  another  rare  type 
of  change  in  the  germ  plasm,  to  which  he  gives  the  name  "duplica- 
tion." Judging  from  the  very  unusual  breeding  results  obtained, 
some  abnormality  in  connection  with  mitosis  has  resulted  in  the 


114  Outline  of  Genetics 

appearance  of  an  extra  piece  of  chromosome  which  dupHcates  in 
its  gene  content  a  known  region  of  one  of  the  normal  chromosomes. 
An  organism  in  this  condition  is  really  triploid  with  respect  to  a 
part  of  one  of  the  chromosome  sets. 

2.  Non-disjunction. — ^This  phenomenon,  made  fa- 
mous through  the  classic  work  of  Bridges  on  the  sex 
chromosomes  of  the  fruit  fly  (chap,  xiii),  may  prove  to 
be  a  fairly  common  occurrence.  In  an  irregular  reduc- 
tion division,  one  of  the  chromosomes  fails  to  '' disjoin" 
properly  from  its  mate.  As  a  result,  one  or  two  gametes 
are  formed  with  an  extra  chromosome,  and  others  which 
lack  this  chromosome.  The  latter  fail  to  function,  but 
a  mating  of  the  former  with  a  normal  gamete  would 
produce  a  zygote  with  an  extra  chromosome.  Blakes- 
LEE,  Belling,  and  Farnham  (2)  have  discovered  this 
phenomenon  in  the  jimson  weed.  Datura.  The  normal 
diploid  number  of  chromosomes  in  this  form  is  twenty- 
four.  Twelve  different  '^ mutants"  have  been  discovered 
with  twenty-five  chromosomes  each.  This  seems  to 
indicate  that  each  of  the  twelve  chromosomes  (haploid) 
has  failed  to  disjoin  at  least  once  in  history.  These 
twelve  new  forms  are  abnormal  in  their  vegetative  fea- 
tures, notably  low  in  fertility,  and  tend  to  revert  to 
the  normal  diploid  ancestor. 

3.  Tetraploidy. — A  hurried  or  incomplete  mitosis 
will  sometimes  result  in  the  simultaneous  duplication  of 
all  of  the  chromosomes.  This  phenomenon  has  been 
observed  several  times  in  culture,  and  there  are  indica- 
tions that  it  has  taken  place  frequently  in  the  past. 
A  general  survey  of  the  chromosome  counts  in  our  exist- 
ing plants  and  animals  emphasizes  the  fact  that  the 
haploid  number  is  much  more  frequently  an  even  number 


Mutation  115 

than  an  odd  one.  This,  together  with  the  fact  that 
there  are  several  species  groups  in  which  the  chromo- 
some count  of  some  of  the  members  is  just  twice  that  of 
the  others,  suggests  that  tetraploidy  may  ha\'e  played 
a  considerable  role  in  evolution.  Tetraploidy  commonly, 
but  not  always,  brings  gigantism. 

Blakeslee  now  puts  the  finishing  touches  on  this 
tetraploidy  conception  by  more  work  on  Datura.  In 
addition  to  the  abnormal  forms  with  twenty-five  chromo- 
somes, he  has  discovered  one  completely  triploid  (thirty- 
six  chromosomes)  and  one  tetraploid  form  (forty-eight 
chromosomes).  These  latter  both  seem  to  be  in  a 
"better-balanced"  condition  than  the  non-disjunctional 
(twenty-five  chromosome)  forms,  since  they  are  more 
''normal"  with  respect  to  their  vegetative  features  and 
fertility.     . 

The  beauty  of  the  situation  arises  from  the  fact  that 
the  tetraploid  type  contains  a  previously  known  Mende- 
lian  factor.  In  normal  diploid  forms  a  hybrid  of  the 
composition  Aa  will  give  a  3:  i  ratio  of  purple  flowered 
and  white  flowered  in  the  F2.  The  tetraploid  hybrid 
AAaa  gives  gametes  in  the  ratio  1  AA:^  AA;  i  aa. 
Chance  matings  among  these  results  in  an  F^  of  35 
purple:  I  white.  The  F3  and  later  generations  behave 
according  to  expectations  on  this  basis. 

As  stated  before,  the  term  "mutation"  is  now  commonly 
restricted  to  locus  changes.  The  author  has  not  discovered  the 
conventional  term  to  include  all  of  the  foregoing  cases  unless  it  be 
merely  "germinal  variations." 

The  bearing  of  these  phenomena  upon  evolution  miglil  be 
considered  briefly.  Until  a  few  years  ago  the  general  belief  on 
evolution  included  the  following  notions:  inheritance  of  acquired 
characters  has  been  exploded;    Darwinian  variations  are  rather 


ii6  Outline  of  Genetics 

dubious  as  a  basis  for  explaining  evolution;  but  mutation,  with 
natural  selection  among  the  mutants,  will  doubtless  account  for 
most  of  the  facts.  Now,  in  view  of  the  more  accurate  knowledge 
of  the  mutation  phenomenon  that  has  been  developed  in  recent 
years,  the  adequacy  of  mutation  in  explaining  evolution  must  be 
considered  more  critically. 

First  of  all,  it  is  evident  that  "complexmutation,"  "defi- 
ciency," and  "duplication"  could  have  played  no  important  part 
in  evolution,  merely  on  account  of  the  extreme  rarity  of  these 
phenomena  if  for  no  other  reason.  Locus  changes  are  sufficiently 
common,  but  consider  the  quahty  of  the  mutants  which  result! 
In  practically  all  cases  the  change  is  a  "loss"  mutation,  and  surely 
evolution  cannot  be  accounted  for  on  such  a  basis!  One  might 
merely  regard  this  as  evidence  of  the  "trial  and  error"  method  by 
which  nature  operates,  only  rarely  making  those  "gains"  which 
must  serve  as  the  basis  of  progressive  evolution.  A  few  "gain" 
mutations  have  been  reported,  but  there  is  reason  to  suspect  that 
even  these  may  be  merely  "reverse"  mutations,  regaining  that 
which  had  previously  been  lost.  Furthermore,  the  locus  changes 
that  have  been  reported,  be  they  losses  or  gains,  have  seemed 
consistently  non-adaptive.  In  short,  it  is  difficult  to  imagine  how 
progressive  evolution  can  be  accounted  for  either  through  single 
locus  changes  or  through  the  accumulation  of  numerous  locus 
changes.  One  can  readily  admit  that  such  changes  may  account 
for  the  multiphcation  of  varieties  or  even  species  "on  the  same 
level,"  but  can  hardly  be  convinced  that  "our  larger  phylogenetic 
edifices  have  been  erected  from  such  building  blocks."  It  is  quite 
likely,  however,  that  our  knowledge  is  still  too  limited  to  visualize 
the  evolution  of  the  ages  in  terms  of  what  we  have  seen  happening 
during  a  very  few  years. 

Non-disjunction  is  out  of  the  question  as  a  basis  for  evolution- 
ary progress.  The  resulting  "unbalanced"  forms  are  clearly 
abnormal,  and  it  is  very  doubtful  whether  they  could  permanently 
perpetuate  themselves  under  the  most  favorable  conditions,  much 
less  survive  under  conditions  of  sharp  competition  and  environ- 
mental stress. 

Tetraploidy  might  well  account  for  a  certain  amount  of  evolu- 
tionary progress,  and  we  have  good  evidence  that  it  has  actually 


Mutation  117 

(lone  so  in  the  past.  'J'he  tetraploid  form  has  more  doses  of  desir- 
able growth  factors  than  had  its  diploid  ancestor,  and  as  a  rule  is 
visibly  more  vigorous  in  one  respect  or  another.  A  theoretical 
limitation,  however,  applies  here  also.  Tetrai)loidy  involves 
merely  a  quantitative  gain,  a  multiplication  of  genes  already 
present.  One  feels  that  not  much  progressive  evolution  could 
take  place  without  the  appearance  of  qualitatively  new  genes  and 
the  production  of  distinctly  new  adaptive  characters. 

It  would  be  safe  to  state  that  the  foregoing  considerations  have 
caused  many  biologists  to  feel  less  certain  in  explaining  evolution 
than  they  were  a  few  years  ago.  This  loss  of  faith  in  mutation, 
taken  together  with  recent  discoveries  on  inheritance  of  acquired 
characters  (see  chap,  ii),  is  causing  many  to  seek  an  explanation 
of  progressive  evolution  in  Lamarckian  terms. 

It  is  of  some  interest  to  note  that  the  original  ''classic" 
examples  of  mutation  provided  by  De  Vries  are  probably  not  gen- 
uine cases.  It  had  long  been  suspected  by  some  that  Oenothera 
Lamarckiana  was  a  hybrid  and  its  ''mutants"  merely  extracted 
recessives,  but  it  was  difficult  to  account  on  this  basis  for  the  very 
small  number  of  "mutants"  that  were  thrown  every  generation. 
MuLLER  (7)  probably  deserves  the  credit  for  solving  this  vexing 
problem.  In  the  fruit  fly  he  discovered  an  essentially  true- 
breeding  hybrid  race  and  explained  it  by  a  system  of  balanced 
lethal  factors.  These  factors  assert  their  lethal  effect  only  when 
they  occur  in  the  homozygous  recessive  condition.  In  this  race 
of  flies,  two  such  factors  are  present  in  heterozygous  condition  on 
the  same  pair  of  chromosomes,  the  dominant  members  of  the 
heterozygous  sets  being  on  the  opposite  chromosomes  of  the  pair. 
Such  a  hybrid  continues  to  breed  true  as  such,  since  any  attempt 
to  segregate  brings  the  homozygous  recessive  condition  of  one  or 
the  other  lethal  with  resulting  death  to  the  progeny.  The  reces- 
sives of  any  heterozygous  set  of  genes  on  this  same  chromo- 
some pair  will  remain  concealed  when  this  stock  is  allowed  to 
inbreed.  Occasional  crossing  over  will  cause  the  appearance  of  a 
few  of  these  recessives  (in  predictable  frequencies),  like  the  "mu- 
tants" thrown  by  Oe.  Lamarckiana. 

In  fact,  De  Vries  himself  now  subscribes  (5)  to  an  explanation 
fundamentally  similar  to  the  preceding.     About  one-half  of  the 


ii8  Outline  of  Genetics 

seeds  of  Oe.  Lamarckiana  are  empty.  De  Vries  explains  this 
by  saying  that  Lamarckiana  produces  two  kinds  of  gametes,  the 
typical  or  laeta  and  the  veliUlna.  Each  gamete  has  a  lethal  factor 
closely  linked  with  the  character  factor.  Heterozygous  combina- 
tions give  good  seeds,  homozygous  give  sterile.  If  one  of  the  two 
lethal  factors  becomes  "vital,"  the  Oe.  laeta  or  Oe.  velutina  muta- 
tion appears. 

LITERATURE  CITED 

1.  Baur,  Erwin,  Mutationen  von  Antirrhinum  majus.  Zeit. 
Induct.  Abstamm.  Vererb.  19:177-193.^^^.  10.  1918. 

2.  Blakeslee,  a.  F.,  Belling,  John,  and  Farnham,  M.  E., 
Chromosomal  duplication  and  Mendelian  phenomena  in 
Datura  mMidMis.     Science  52:388-390.  1920. 

3.  Bridges,  Calvin  B.,  \'ermilion-deficiency.  Jour.  Gen. 
Physiol.  1:645-656.  1919. 

4. ,  The  developmental  stages  at  which  mutations  occur 

in  the  germ  tract.     Proc.  Soc.  Exp.  Biol,  and  Med.  17:1-2. 
1919. 

5.  De  Vries,  Hugo,  Phylogenetische  und  gruppenweise  Artbil- 
dung.     Flora  11-12:208-226.  1918. 

6.  Morgan,  T.  H.,  Evolution  by  mutation.  Sci.  Monthly  5: 
46-53.  1918. 

7.  MuLLER,  H.  J.,  Genetic  variability,  twin  hybrids,  and  con- 
stant hybrids,  in  a  case  of  balanced  lethal  factors.  Genetics 
3: 42 2-4gg.  fig.  I.  1918. 

8.  ,  Further  changes  in  the  white-eyed  series  of  Drosophila 

and  their  bearing  on  the  manner  of  occurrence  of  mutation. 
Jour.  Exp.  Zool.  31:443-473.^^.  3.  1920. 

9.  — -,   Variations   due   to  change  in  the  individual   gene. 

Amer.  Nat.  56:32-50.  1922. 

10.  — ,  and  Altenburg,  E.,  A  study  of  the  character  and 

mode  of  origin  of  eighteen  mutations  in  the  X  chromosome  of 
Drosophila.     Anat.  Rec.  20:213.  1921. 

11.  Nilsson-Ehle,  H.,  Multiple  allelomorphe  und  Komplexmu- 
tationen  beim  Weizen.     Hereditas  1:227-311.  1920. 

12.  Zeleny,  Charles,  The  direction  and  frequency  of  mutation 

in  a  series  of  multiple  allelomorphs.     Anat.  Rec.  20:210-211. 
1921. 


CHAPTER  VIII 

BUD  VARIATION 

The  outstanding  feature  of  bud  variation  is  that  we  know  very 
little  about  it.  It  is  true  that  quite  a  number  of  cases  of  bud 
variation  have  been  investigated,  but  it  could  hardly  be  said  that 
altogether  satisfactory  explanations  of  the  phenomena  have  as  yet 
been  provided.  The  relation  between  bud  variation  and  the 
JVIendelian  mechanism  is  difficult  to  visualize  with  much  clearness, 
nor  is  it  easy  to  interpret  the  various  cases  in  terms  of  each  other. 

Bud  variation  may  be  defined  as  variation  originating  in  vege- 
tative tissue.  Such  variation  might  involve  merely  (i)  'fluctua- 
tion," a  response  to  environmental  stimulus,  or  it  might  involve 
(2)  a  change  in  the  genetic  constitution  of  the  parts  affected. 
Cases  of  type  (i)  need  not  concern  us  here,  since  such  variations 
are  not  inherited.  As  for  type  (2),  this  should  be  subdivided  into: 
(a)  cases  in  which  the  variation  involves  both  somatic  and  germinal 
tissue,  and  in  which,  therefore,  the  variation  will  be  heritable 
through  seed;  and  (b)  cases  in  which  the  variation  involves  somatic 
tissue  alone,  the  variation  not  being  heritable  through  seed.  With 
these  distinctions  in  mind,  we  may  consider  a  classification  of  the 
phenomena  of  bud  variation,  which  is  based  primarily  upon  the 
ideas  of  Emerson  (6). 

I.      SOMATIC   MUTATION   OF   GENES 

This  may  be  illustrated  by  some  of  the  findings  of  Emerson 
in  corn.  An  illustration  of  variation  involving  both  somatic  and 
germinal  tissue  is  provided  by  the  behavior  of  pericarp  color.  5 
is  a  gene  which  results  in  self-  (completely)  colored  grains,  being 
dominant  to  V  which  produces  variegated  grains,  and  which  in 
turn  is  dominant  to  W  which  produces  colorless  grains.  (These 
are  three  members  of  a  series  of  "multiple  allelomorphs";  see 
p.  112.)  Corn  of  the  formula  VW,  and  which  should,  therefore, 
have  all  the  grains  variegated,  will  at  times  have  some  grains  that 

119 


I20  Outline  of  Genetics 

are  self-colored.  Further  breeding  reveals  that  somatic  mutation 
has  occurred  in  the  tissues  concerned  with  the  formation  of  these 
grains,  such  that  VW  has  become  SW.  The  change  has  taken 
place  not  only  in  the  somatic  tissue  of  the  grains  in  question,  but 
also  in  the  germinal  tissue  within  the  grains,  and  is  inherited 
accordingly.  It  is  concluded  that  the  recessive  variegation  gene 
V  has  mutated  to  its  dominant  self-color  allelomorph  S. 

Without  attempting  a  discussion  of  the  breeding  tests  upon 
which  EiviERSON  based  his  conclusions,  it  will  be  worth  while  at 
least  to  mention  some  of  the  other  peculiarities  of  this  phenome- 
non. V  mutates  to  5"  rather  frequently,  but  IF  never  mutates 
to  S.  V  in  the  heterozygous  condition  (VW)  mutates  to  S  five 
times  as  frequently  as  when  it  is  in  homozygous  combination  (VV). 
The  mutation  takes  place  late  in  ontogeny  much  more  often  than 
in  early  ontogeny.  As  a  result  there  are  many  more  cases  where 
small  patches  of  self-colored  grains  appear  on  variegated  ears  than 
where  large  patches  appear.  In  corn  of  the  VV  formula,  only 
one  of  the  V  genes  ever  mutates  to  5  at  a  given  time.  Reverse 
mutations,  S  changing  to  F,  have  also  been  noted. 

This  same  material  provides  also  an  example  of  somatic  muta- 
tion which  involves  the  soma  alone  and  not  the  germinal  tissue. 
In  situations  essentially  similar  to  those  described  above,  there 
may  appear  on  the  variegated  ears  a  few  aberrant  grains  which  are 
apparently  self-colored  only  on  the  crown  of  the  seed.  This 
character  has  been  designated  as  "dark-crown,"  and  it  is  notable 
that  it  is  never  inherited.  Microscopic  examination  of  the  dark- 
crown  and  of  the  fully  self-colored  seeds  indicates  that  in  the  former 
the  epidermis  alone  is  colored,  while  in  the  latter  the  epidermis 
alone  remains  colorless.  The  conclusion  seems  warranted,  there- 
fore, that  the  two  types  of  variation  are  fundamentally  the  same, 
both  being  true  gene  mutations,  and  that  the  non-inheritance  of 
the  dark-crown  type  is  due  to  the  accident  that  it  occurs  in  the 
epidermal  tissue  outside  the  germ  tract. 

II.      SOMATIC   SEGREGATION 

It  has  been  pointed  out  by  several  investigators  that  bud 
variations  appear  much  more  frequently  in  plants  that  are  hetero- 
zygous for  the  genes  concerned  than  in  plants  which  are  homozy- 


Bud  Variation 


121 


gous.  This  may  be  true,  but  it  would  not  be  safe  to  conclude  that 
there  is  any  mechanism  ever  provided  in  somatic  tissue  which 
corresponds  to  the  normal  Mendelian  mechanism  for  segregation 
during  gametogenesis.  If  "somatic  segregation"  ever  takes 
place,  it  is  through  the  operation  of  some  quite  different  mecha- 
nism, as  indicated  in  the  examples  given  below. 

1.  Chromosome  elimination. — A  theoretical  illustration 
would  be  as  follows.  A  plant  heterozygous  for  linked  genes,  A-B 
a-b,  has  an  irregular  mitosis  take  place  in  some  part  of  its  somatic 
tissue.  One  of  the  daughter-nuclei  of  this  mitosis  fails  to  receive 
its  full  complement  of  chromosomes,  the  A-B  chromosome  hav^ing 
somehow  been  eliminated.  This  cell  and  its  progeny,  haploid  now 
with  respect  to  this  chromosome  pair,  which  is  represented  only  by 
the  a-b  chromosome,  will  form  tissue  in  which  the  recessive  char- 
acters a  and  b  will  become  manifest. 

This  would  be  the  principle  underlying  somatic  segregation 
through  chromosome  elimination.  As  a  matter  of  fact,  there  are 
really  only  two  clearly  demonstrated  cases  of  this  sort,  and  both 
of  these  are  limited  to  rather  special  situations.  In  one  of  these 
cases,  "  gynandromorphism "  in  the  fruit  fly,  a  special  chromo- 
some set  is  involved,  the  sex  chromosomes.  This  will  be  taken  up, 
therefore,  in  the  chapter  on  "Sex  determination."  In  the  other 
case,  endosperm  "mosaics"  in  corn,  a  special  triploid  tissue,  the 
endosperm,  is  involved.  This  case  will  be  discussed  in  the  chap- 
ter on  "The  endosperm  in  inheritance." 

2.  Chimaeras. — A  chimaera  is  a  plant  in  which  some  of  the 
tissues  have  all  of  the  characteristics  of  one  variety  or  species, 
while  the  rest  of  the  tissues  on  this  same  plant  are  characteristically 
those  of  a  different  variety  or  species.  The  most  famous  chimae- 
ras are  the  "graft-hybrids"  of  Solanum  produced  by  W'inki.kk 
(lo).  This  investigator  made  grafts  of  two  distinct  species  of 
this  genus,  the  tomato  and  the  nightshade.  After  the  tissues  of 
stock  and  scion  had  been  given  time  to  "weld"  together,  Wink- 
ler cut  the  stem  in  such  a  way  that  the  exposed  cross-section 
was  made  up  partly  of  tissues  of  the  stock  and  partly  of  scion 
tissues.  From  this  cut  surface,  adventitious  buds  would  arise, 
and  at  times  these  buds  came  at  the  exact  point  where  stock  and 
scion  tissues  were  in  contact.   Such  buds  developed  branches  which 


122  Outline  of  Genetics 

were  sectoral  chimaeras,  the  tissues  of  one  side  of  the  branch  being 
those  of  the  tomato,  while  the  tissues  of  the  other  side  of  the  branch 
were  night-shade  tissues.  Such  sectoral  chimaeras  would  not 
infrequently  later  produce  branches  that  were  periclinal  chimaeras, 
having  the  tissues  of  one  species  inclosed  within  an  envelope  of  the 
other.  That  these  were  really  periclinal  chimaeras  was  established 
by  chromosome  counts  (tomato  and  nightshade  having  different 
chromosome  numbers),  and  by  the  fact  that  seedlings  produced 
by  them  were  always  of  the  species  of  the  subepidermal  tissue 
from  which  the  gametes  arise.  The  periclinal  chimaeras  in  turn 
were  observed  at  times  to  produce  branches  wholly  of  one  or  the 
other  of  the  parent-species,  a  performance  which  may  well  be 
regarded  as  a  type  of  somatic  segregation. 

Fundamentally,  the  same  behavior  has  been  observed  in 
certain  "natural"  periclinal  chimaeras  (notably  in  types  of  Pelar- 
gonium, Baur  2),  involving  white  (deficient  in  chlorophyll)  and 
green  tissues.  The  manner  of  origin  of  these  natural  chimaeras 
is  unknown,  but  it  is  quite  possible  that  they  arose  as  somatic 
mutations. 

A  very  interesting  case  has  been  reported  by  Bateson  (3)  in 
Bouvardia,  which  presumably  may  be  something  of  the  same  sort 
as  the  foregoing.  Varieties  of  Bouvardia  that  are  maintained  true 
to  type  by  propagations  from  stem  cuttings  produce  plants  with 
very  different  flower  form,  size,  and  color  when  propagated  by  root 
cuttings.  Since  in  normally  produced  buds  of  the  stem  both  the 
epidermis  and  the  deeper  lying  tissues  are  maintained  through 
direct  cell  lineage,  while  the  roots  produced  by  stem  cuttings  arise 
from  the  plerome  and  break  through  the  periblem  and  dermatogen, 
forming  these  parts  anew,  sprouts  that  develop  from  the  roots 
must  have  the  genotype  of  the  stele  rather  than  that  of  the  cortex 
or  epidermis. 

It  is  clear  that  classes  i  and  2  represent  distinct  types  of 
somatic  segregation,  the  first  arising  as  the  result  of  irregular  chro- 
mosome distribution  and  the  second  from  a  segregation  on  the 
part  of  tissues  as  a  whole.  Both  might  well  be  regarded  as  anom- 
alies, since  they  are  to  be  explained  by  irregularities  in  the  com- 
mon plant  program.  There  remains  to  be  considered  one  more  type 
of  somatic  segregation,  and  here,  although  no  such  finely  balanced 


Bud  Variation  123 

mcchaaism  for  segregation  is  involved  as  that  of  the  reduction 
division,  at  least  the  segregation  is  effected  with  some  regularity. 
3.  Cytoplasmic  segrkgation.— Numerous  cases  of  apparent 
segregation  of  cytoplasmic  elements  have  been  i)rovi(led  in  i)lants. 
All  of  them  involve  visible  effects  on  chlorophyll  and  all  show  non- 
Mendelian  inheritance.  (Cytoplasmic  segregation  is  the  conclud- 
ing item  in  Emerson's  classification  of  bud  variation.  In  order 
better  to  bring  out  the  relationship  between  cytoplasmic  segrega- 
tion and  certain  other  plant  phenomena,  this  item  will  be  taken 
up  as  a  part  of  the  following  classification  [from  Winge  9]  of  cases 
of  chlorophyll  inheritance.) 

Chlorophyll  Lvheritaxce 

I.  IMendelian  inheritance,  the  characters  being  "carried  in" 
the  nucleus. 

Quite  an  array  of  cases  of  chlorophyll  deficiency  have  been 
found  to  be  inherited  according  to  the  normal  JMendelian  mecha- 
nism. In  this  class  have  been  noted  albino,  pale  green,  yellow,  and 
variegated  types  which  are  (usually)  inherited  as  simple  Mendelian 
lecessives  to  the  normal  green  condition. 

II.  Non-Mendelian  inheritance,  the  characters  being  carried 
in  some  extra-nuclear  portion  of  the  gametes  (Emerson's  cyto- 
plasmic segregation). 

I.  Biparental  inheritance,  the  male  as  well  as  the  female  par- 
ent contributing  (presumably)  both  cytoplasm  and  plastids  to  the 
zygote. 

A.  The  chlorophyll  character  governed  by  the  distribution  of 
the  plastids  themselves. 

An  example  of  this  type  of  thing  is  provided  by  the  work  of 
Baur  (2)  on  certain  types  of  Pelargonium,  where  the  following 
behavior  has  been  noted.  If  a  white-leaved  plant  (white-leaved 
branch,  see  below)  and  a  normal  green-leaved  plant  are  crossed 
(either  way),  the  resulting  hybrid  illustrates  what  has  been  called 
by  some  ''particulate  inheritance";  that  is.  the  hybrid  is  varie- 
gated, showing  irregular  patches  of  green  and  white.  If  one  of 
these  white  patches  completely  includes  a  bud.  there  will  probably 
be  produced  by  that  bud  a  completely  white  branch.  The  flowers 
of  this  branch,  when  self-fertilized,  give  rise  through  their  seeds 


124  Outline  of  Genetics 

to  white  individuals  only,  and  would  evidently  continue  to  breed 
true  to  the  white  condition  if  white  individuals  could  be  matured. 
In  like  manner  the  variegated  hybrid  may  give  rise  to  a  pure  green 
branch,  which  would  start  a  line  of  pure  green  individuals. 

A  suggested  explanation  of  this  situation  is  that  the  white 
condition  results  from  the  occurrence  of  purely  colorless  plastids 
in  the  tissue,  while  the  green  condition  has  the  normal  green 
chloroplasts.  A  cross  between  the  two  types  will  introduce  into 
the  hybrid  zygote  a  mixture  of  green  and  white  plastids  ("plastid 
primordia");  and  the  same  result  will  be  obtained  whichever  way 
the  cross  is  made,  since  the  male  as  well  as  the  female  parent  con- 
tributes plastids  to  the  zygote.  During  somatogenesis  in  the  plant 
which  develops  from  such  a  zygote,  there  will  be  an  inevitable 
segregation  of  green  and  white  plastids,  since  there  is  no  mecha- 
nism provided  for  a  perfectly  even  distribution  to  daughter-cells 
of  those  cell  components  which  lie  outside  the  nucleus.  If  the 
number  of  plastids  per  cell  be  not  too  large,  sooner  or  later, 
through  the  operation  of  the  laws  of  chance,  cells  will  arise  which 
contain  plastids  entirely  of  one  sort  or  the  other,  and  these  will 
produce  tissues  which  are  pure  green  or  pure  white. 

B.  The  chlorophyll  character  governed  by  the  distribution 
of  other  and  finer  cytoplasmic  elements  than  the  plastids  them- 
selves. 

Again  male  and  female  parents  both  contribute  the  effective 
extra-nuclear  elements  to  the  hybrid  zygote,  and  again  a  tendency 
tow^ard  irregular  segregation  appears  during  somatogenesis  of  the 
resulting  plant.  In  this  case,  however,  the  effective  units  are  so 
small  and  numerous  that  a  complete  segregation  of  units  of  the 
two  types  is  never  achieved,  but  merely  the  production  of  rela- 
tively paler  and  relatively  greener  regions  on  the  plant. 

The  behavior  of  Ikeno's  (7)  albomaculaia  type  of  Capsicum 
might  be  interpreted  on  this  basis.  Paler  and  greener  patches 
occur  on  the  albomaculata  plants,  and  the  average  paleness  of  the 
whole  individual  may  be  greater  or  less.  Since  this  "average 
paleness"  of  the  parent  is  reflected  in  the  nature  of  the  offspring, 
from  any  sort  of  a  cross,  it  is  felt  that  such  a  parent  produces 
gametes  having  a  characteristic  proportion  of  green  and  white 
elements  or  units,  whatever  these  elements  or  units  may  be. 


Bud  Varialio}i  125 

2.  Maternal  inheritance,  the  male  parent  contributing  only 
a  nucleus  to  the  make-up  of  the  zygote. 

In  cases  of  this  sort  the  source  of  pollen  makes  no  dilTerence 
so  far  as  the  chlorophyll  character  is  concerned.  Consequently, 
since  this  maternal  inheritance  cannot  be  accounted  for  by  par- 
thenogenesis in  the  plants  that  were  used,  it  has  been  concluded 
that  the  seat  of  the  character  in  question  is  in  something  that  the 
female  parent  regularly  contributes  and  the  male  parent  never 
contributes.  This  idea  is  supported  by  certain  cytological  evidence 
that  suggests  the  fact  that,  in  some  plants  at  least,  the  male 
nucleus  is  '' stripped  clean  of  its  cytoplasm"  at  the  time  that  it  is 
discharged  from  the  pollen  tube  into  the  embryo  sac.  A  similar 
distinction  to  that  made  under  i  may  also  be  applied  here. 

A.  The  chlorophyll  character  governed  by  the  distribution  of 
the  plastids  themselves. 

It  follows  that  pure  green  and  white  parts  will  at  times  segre- 
gate out  in  the  variegated  plants  concerned.  Correxs'  (5) 
albomaculata  type  of  Mirabilis  is  said  to  be  an  example  of  this  sort 
of  thing. 

B.  The  chlorophyll  character  governed  by  the  distribution  of 
other  and  finer  cytoplasmic  elements  than  the  plastids  themselves. 

It  follows  that  absolutely  pure  green  and  pure  white  parts  will 
never  segregate  out,  but  only  relatively  paler  and  relatively  greener 
parts.  WiNGE  (9)  cites  some  of  his  own  work  on  a  variegated 
type  of  Humulus  as  an  example. 

In  conclusion  it  should  be  said  that  opinion  as  to  the  seat  of 
chlorophyll  inheritance  is  by  no  means  settled.  It  would  doubtless 
be  wise  to  regard  the  foregoing  classification  of  W'in'ge's  merely 
as  a  convenient  form  in  which  to  arrange  the  available  evidence; 
other  investigators  would  certainl}'  disagree  with  some  of  Win'ge's 
interpretations  of  the  phenomena. 

As  an  example  of  a  case  which  can  liardly  be  forced  into 
Winge's  classification,  An'dersox's  (i)  green  and  white  variegated 
race  of  corn  might  be  cited.  Inheritance  is  strictly  maternal,  and 
pure  green  and  white  areas  segregate  out  on  tlie  leaves.  Presum- 
ably then  this  would  fit  into  Win'ge's  class  II,  2.  A.  Hut  a  care- 
ful cytological  investigation  of  this  material  by  Randolph  (8)  has 
revealed  that  there  can  be  no  sharp  segregation  among  green  and 


126  Outline  of  Genetics 

white  plastids  at  play  to  account  for  the  green  and  white  areas; 
that  it  must  be  rather  a  matter  of  the  ''general  physiological 
condition  "  of  the  two  types  of  tissue.  It  is  interesting  to  note  that 
in  this  case  a  type  of  somatic  segregation  occurs  on  the  ear  of  the 
variegated  plant,  resulting  in  certain  groups  of  seeds  that  will 
produce  green  seedlings,  other  groups  of  seeds  that  will  produce 
white  seedlings,  and  still  other  groups  of  seeds  that  will  produce 
variegated  seedlings. 

In  short,  the  cases  of  chlorophyll  inheritance  on  a  non-Men- 
delian  basis  are  still  under  considerable  discussion;  a  perfectly 
clear  interpretation  of  the  phenomena  is  not  as  yet  available.  Of 
this  much,  however,  we  may  be  sure:  there  is  such  a  thing  as  non- 
Mendelian  inheritance,  and  it  becomes  manifest  in  connection 
with  a  type  of  character  which,  on  other  occasions,  is  inherited 
according  to  the  normal  Mendelian  scheme.  In  any  event,  such 
cases  should  not  be  regarded  as  a  violation  of  Mendel's  law,  but 
merely  as  something  outside  the  scope  of  Mendel's  law,  since  they 
are  evidently  transmitted  by  some  extra-nuclear  mechanism. 

In  good  part  the  known  examples  of  non-Mendelian  inherit- 
ance are  limited  to  such  cases  of  chlorophyll  inheritance  as  have 
been  cited  above.  There  is  another  small  group  of  cases,  however, 
that  must  also  be  regarded  as  illustrating  non-Mendelian  inherit- 
ance, although  in  quite  a  different  way.  Bateson  and  his 
coworkers  (4)  have  discovered  certain  instances  (e.g.,  inheritance 
of  doubleness  in  Matthiola)  in  which  the  male  and  female  organs 
of  the  same  plant  differ  in  the  factors  they  carry.  A  clear  explana- 
tion of  this  phenomenon  has  not  been  provided,  but  whatever 
the  explanation  may  turn  out  to  be,  it  seems  certain  that  it  will 
provide  an  exception  to  the  normal  Mendelian  mechanism.  Such 
cases  have  led  Bateson  to  suspect  that  plants,  as  genetic  machines, 
differ  fundamentally  from  animals,  segregation  being  clearly 
connected  with  synapsis  in  animals  but  not  always  in  plants. 
This  difference  in  the  machinery  may  be  tied  up  with  the  fact  that 
"in  animals  the  rudiments  of  the  gametes  are  often  visibly  sepa- 
rated at  an  early  embryonic  stage,  whereas  in  the  plant  they  are 
given  off  from  persistent  growing  points." 


Bud   Variation  127 

LITERATURE  CITED 

1.  Anderson,  E.  G.,  Maternal  inheritance  of  chlorophyll  in 
maize.    Unpublished. 

2.  Baur,  E.,  Das  Wesen  und  die  Erblichkeitsverhiiltnisse  tier 
''Varietates  albomarginatae  hort"  von  Pelargonium  zonule. 
Zeit.  Abst.  und  Vererb.-lehre  1:330-351.  1908. 

3.  Bateson,  W.,  Root-cuttings,  chimaeras  and  ''sports."  Jour. 
Genetics  6: 75-80.  1916. 

4.  ,  The  progress  of  Mendelism.     Xature   104:214-216. 

1919. 

5.  CoRRENS,  C,  \'crerbungsversuche  mit  blass  (gelb)  griincn  und 
buntblatterigen  Sippen  bei  Mirabilis  Jala  pa,  Urtica  pilulijcra 
und  Lunaria  annua.  Zeit.  Abst.  und  Vererb.-lehre  1:291- 
299.  1908. 

6.  Emerson,  R.  A.,  The  nature  of  bud  variations  as  indicated  by 
their  mode  of  inheritance.     Amer.  Nat.  56:64-79.  1922. 

7.  Ikeno,  S.,  Studies  on  the  hybrids  of  Capsicum  annuum. 
Part  11.  On  some  variegated  races.  Jour.  Genetics  6:201- 
229.  1917. 

8.  Randolph,  L.  F.,  Cytology  of  chlorophyll  types  of  maize. 
Bot.  Gaz.  73:337-375.  pis.  11-16.  1922. 

9.  WiNGE,  O.,  On  the  non-Mendelian  inheritance  in  variegated 
plants.     Compt.  Rend.  Carlsberg  14:1-20.  1919. 

10.  Winkler,  H.,  Die  Chimarenforschung  als  IMethodc  der  experi- 
mentellen  Biologie.  Phys.  IMed.  Gesell.  Wiirzburg.  Jahrb. 
1913,  1914. 


CHAPTER  IX 

THE  GAMETOPHYTE  IN  INHERITANCE 

Thus  far  the  discussions  have  dealt  with  inheritance  in  sporo- 
phytes;  in  fact,  genetics  practically  never  considers  ganietophytes, 
through  which  inheritance  must  pass  from  one  sporophyte  to  the 
next.  The  reasons  for  this  neglect  are  obvious.  Practically  all 
of  our  land  vegetation  is  made  up  of  sporophytes,  and  therefore 
practically  all  of  our  experimental  material  has  been  sporophytes. 
Furthermore,  gametophytes  are  inconspicuous  (out  of  sight  in 
seed  plants),  hard  to  get  at,  hard  to  work  with,  and  apparently  of 
no  economic  importance.  Besides,  in  animals,  as  is  well  known, 
the  generation  equivalent  to  the  gametophyte  of  plants  is  repre- 
sented by  only  a  few  cell  divisions  in  the  maturation  of  gametes. 
In  other  words,  the  gametophyte  has  no  significance  as  a  discrete 
generation  in  the  animal  kingdom;  and  since  inheritance  in  plants 
is  of  interest  to  the  public  chiefly  because  it  throws  some  light  upon 
inheritance  in  animals,  there  has  been  little  demand  for  any  knowl- 
edge of  inheritance  in  gametophytes. 

It  is  not  surprising,  therefore,  that  very  little  study  has  been 
made  of  the  gametophyte  generation  in  inheritance.  There  are 
reasons  for  believing,  however,  that  such  a  study  might  be  very 
profitable.  The  gametophyte  generation,  with  its  haploid  chromo- 
some number,  would  provide  an  interesting  and  critical  test  of  the 
Mendehan  mechanism  of  inheritance.  Certain  features  of  inherit- 
ance would  be  expected  to  differ  radically  from  inheritance  in 
sporophytes.     A  generalized  example  might  be  considered. 

Gametophyte  Ab  is  characterized  by  exhibiting  the  A  character 
but  not  the  B.  The  reverse  is  true  of  gametophyte  aB.  A  cross 
between  the  two  would  produce  zygote  AaBb,  followed  by  a  sporo- 
phyte of  the  same  formula.  The  following  gametophyte  genera- 
tion would  contain  four  types  in  equal  numbers,  AB,  Ab,  aB,  and 
ab.  This  is  the  program  that  would  have  to  be  followed  if  the 
Mendelian  mechanism  were  at  play.     One  would  expect,  there- 

128 


The  Gamelophyte  in  Inhentance  129 

fore,  that  there  would  be  the  following  outstanding  character- 
istics of  inheritance  in  ganietophytes.  (i)  There  could  be  no 
possibility  of  dominance,  since  but  one  representative  of  an  allclo- 
morphic  pair  could  be  present.  Any  discovery  of  blending  inherit- 
ance in  gametophytes  (with  respect  to  a  monoh\'brid  situation) 
would  serve  to  cast  doubt  upon  the  Mendelian  mechanism.  (2) 
Clear  segregation  would  appear  in  the  first  generation  following 
the  cross  and  the  phenot^-pic  ratios  would  be  quite  different  from 
those  encountered  among  sporophytes.  'I'he  various  factor  t>pes 
might,  of  course,  be  expected  to  operate,  but  again  the  ratios 
obtained  would  be  unusual.  All  of  this,  however,  is  little  more 
than  idle  speculation,  serving  merely  to  point  out  discoveries  that 
might  be  expected  in  the  future. 

A  study  of  inheritance  in  gametophytes  might  be  profitable 
for  other  reasons  also.  Among  those  higher  plants  which  have 
been  the  objects  of  genetical  research,  the  sex  act  is  a  very  well- 
insulated  performance,  occurring  deep  within  the  tissues.  Fur- 
thermore, it  is  both  prefaced  and  followed  by  quite  a  sequence  of 
events,  which  we  know  must  be  taking  place  with  considerable 
regularity  but  which  we  cannot  control.  Surely  there  would  be 
much  greater  hope  of  any  artificial  manipulation  of  the  sex  act, 
making  possible  a  more  critical  study  of  the  germ  plasm,  in  those 
organisms  where  the  gametes  themselves  could  be  manipulated. 
It  would  seem  that  such  things  might  be  possible  in  those  lower 
plants  where  the  gametophyte  is  the  dominating  generation, 
although  the  technique  necessary  for  such  experiments  would 
doubtless  be  difficult  to  develop. 

The  actual  work  that  has  been  done  on  inheritance  in  game- 
tophytes is  practically  nil.  Pure  line  studies  have  been  made  in 
a  number  of  thallophytes,  selection  has  been  attempted,  and 
some  mutations  have  been  found,  but  none  of  the  experiments 
has  revealed  anything  critical  on  the  matter  of  segregation  of 
characters  following  a  sex  act. 

Transeau  (2)  has  made  some  observations  on  ll\c  green  alga 
Spirogyra,  which,  while  they  did  not  involve  any  experimental 
work,  were  nevertheless  quite  suggestive.  This  author  was  famil- 
iar with  several  species  of  Spirogyra  in  their  natural  habitats,  and 
noted  several  natural  populations  which  were  clearly  mixtures  of 


130  Outline  of  Genetics 

two  or  more  different  species.  Under  such  circumstances  fila- 
ments of  one  species  were  observed  in  the  act  of  crossing  with 
filaments  of  another  species.  Furthermore,  in  such  mixed  popu- 
lations there  were  discovered  some  filaments  which  were  evidently 
the  products  of  previous  crosses,  for  they  clearly  showed  new  com- 
binations of  the  characters  of  two  species.  It  is  to  be  hoped  that 
exact  experimental  work  will  prove  feasible  with  this  genus. 

One  instance  of  the  clean  segregation  of  characters  in  the 
gametophyte  generation  is  to  be  found  among  angiosperms, 
Selling's  semi-sterility  among  beans.  This  wdll  be  described 
later  in  the  discussion  of  the  general  subject  of  sterility.  For  the 
most  part  the  gametophyte  of  angiosperms  seems  to  be  merely 
an  expressionless  intermediate  stage  between  succeeding  sporo- 
phyte  generations.  As  East  (i)  puts  it:  ''Modern  discoveries 
tend  more  and  more  to  show  that  the  sole  function  of  the  gameto- 
phyte of  the  angiosperms  is  to  produce  sporophytes.  The  char- 
acters which  they  carry  appear  to  be  wholly  sporophytic,  the 
factors  which  they  carry  functioning  only  after  fertilization." 

LITERATURE  CITED 

1.  East,  E.  M.,  and  Park,  J.  B.,  Studies  on  self-sterility.     I. 
The  behavior  of  self-sterile  plants.     Genetics  2:505-609.  191 7. 

2.  Transeau,  E.  N.,  Hybrids  among  species  of  Spirogyra.     Amer. 
Nat.  53:109-119.^^5.  7.  1919. 


CHAPTER  X 

STERILITY 

The  subject  of  sterility  in  plants  is  only  in  part  a  matter  of 
genetics.  Many  of  the  problems  involved  can  be  solved  only  by 
the  physiologist,  ecologist,  or  cytologist.  Some  phases  of  the 
subject,  however,  have  been  rather  successfully  interpreted  in 
terms  of  genetics.  Tentative  outlines  of  the  general  subject  will 
be  presented,  merely  to  show  what  parts  of  the  problem  are  being 
attacked  by  the  geneticist. 

First  of  all,  sterility  might  be  classified  in  terms  of  the  effects 
produced : 

A.  Sterility.     Complete  failure  of  the  sex  act. 

B.  Semi-sterility.  Failure  of  part  of  the  pollen,  or  part  of 
the  ovules,  or  part  of  both. 

C.  Self-sterility.  Pollen  and  ovules  functional  in  cross- 
fertilization  but  not  in  self-fertilization. 

A  more  comprehensive  classification  might  then  be  arranged 
on  the  basis  of  cause,  although  such  a  classification,  in  our  present 
state  of  knowledge,  must  be  rather  vague  and  uncertain. 

I.  Environmental  causes.  (Merely  a  few  examples  will  be 
indicated.  This  part  of  the  subject  properly  belongs  to  the 
ecologist  and  physiologist.) 

The  examples  of  environmental  causes  given  below  commonly 
result  in  complete  sterility  A,  although  under  special  circumstances 
situations  corresponding  to  B  or  C  might  be  set  up. 

I.  Conditions  too  moist. 

When  species  that  have  become  adapted  to  relatively  dry 
conditions  are  subjected,  at  the  lime  pollen  is  mature  and  shedding, 
to  unusually  moist  conditions,  the  pollen  grains  may  absorb 
enough  moisture  to  swell  up  and  burst  prematurely,  thus  losing 
their  usefulness.  (The  sex  act  might  also  be  circumvented  by 
hard  rain  coming  immediately  after  pollen   distribution,   which 

131 


132  Outline  of  Genetics 

would  wash  the  pollen  grains  off  the  stigmas,  and  thus  result  in  a 
certain  amount  of  sterility.) 

2.  Conditions  too  dry. 

Unusual  drought  at  the  period  that  the  stigmas  are  receptive 
may  dry  the  stigmatic  surfaces  to  such  a  degree  that  pollen  grains 
will  not  adhere  and  germinate.  Conditions  of  this  sort  at  times 
limit  the  setting  of  seed  in  such  plants  as  corn. 

I  and  2  provide  an  illustration  of  a  principle  which  is  frequently 
encountered  in  biology^ — -opposite  extremes  of  conditions  bringing 
a  similar  end  result  (although  the  intervening  sequences  of  causes 
and  effects  will  of  course  differ).  Other  illustrations  of  this  same 
thing  appear  later  in  this  classification. 

3.  Poor  ''nutrition." 

This  may  so  limit  the  development  of  plants  that  they  fail  to 
mature  up  to  the  point  of  effecting  a  sex  act. 

4.  Good  ''nutrition." 

A  principle  familiar  to  botanists  is  that  the  optimum  growth 
conditions  frequently  maintain  the  plant  in  the  purely  vegetative 
phase,  so  that  reproductive  parts  are  not  developed.  (The  plant 
physiologist  describes  this  in  terms  of  a  carbohydrate :  nitrogen 
ratio.) 

5.  Season  too  short. 

Plants  adapted  to  a  long  growing  season  are  unable  to  com- 
plete their  normal  life-cycle  up  to  the  point  of  successful  reproduc- 
tion when  grown  in  regions  which  have  a  short  season. 

6.  Unusual  light  conditions. 

Flowering  and  fruiting  of  many  kinds  of  plants  is  induced  by 
exposure  to  specifically  favorable  length  of  day  which  varies  widely 
with  the  species  (see  Allard  and  Garner  i).  Radical  departures 
from  the  customary  seasonal  program  in  this  matter  may  serve 
to  inhibit  flowering  and  fruiting.  (It  may  be  that  the  underlying 
causes  involved  here  are  similar  to  those  of  3  and  4.) 

Other  examples  of  environmental  causes  for  sterility  could 
doubtless  be  provided  by  the  plant  ecologist. 

II.  Large  evolutionary  tendencies. 

A.  Sterility. 

Within  certain  groups  of  plants,  what  is  apparently  the 
natural   phylogenetic   sequence   of  genera   and   species   indicates 


Sterility  i  t,2> 

evolutioiuiry  progress  in  the  dirccLion  of  cslablisliiiig  partheno- 
genesis. (The  most  notable  example  is  i)rovi(le(l  by  the  C'om- 
positae,  although  it  would  certainly  be  unsafe  to  conclude 
therefrom  that  the  ultimate  goal  of  plant  evolution  was 
parthenogenesis.  Quite  on  the  contrary,  one  feels  that  the 
Compositae  have  seriously  handicapped  themselves,  so  far  as 
future  possibilities  are  concerned,  by  a  ''freakish''  evolutionary 
maneuver.) 

C.  Self-sterility. 

In  effect  evolution  among  angiosperms  has  achieved  self- 
sterility  through  the  perfection  of  devices  favoring  cross- 
pollination.  Here  should  be  cited:  floral  adjustments  insuring 
cross-pollination  by  insects;   protandry  and  protogeny;    dioecism. 

(B.  Semi-sterility. 

This  class  has  little  more  than  a  theoretical  existence  here, 
although  doubtless  some  forms  might  be  found  exhibiting  the 
''incompletion"  of  some  of  the  above-mentioned  evolutionary 
tendencies,  and  thereby  exhibiting  what  is  in  effect  semi-sterility.) 

III.  Phenomena  of  genetics. 

Here  are  included  cases  where  the  mechanism  underlying  the 
phenomenon  is  affected  by  breeding  operations. 

A.  Sterility. 

I.  Wide  crosses. 

Crosses  between  distantly  related  parents  may  be  effective 
in  producing  first  generation  hybrids  which  may  be  notably 
vigorous  individuals  (see  chapter  on  ''Hybrid  vigor"),  but  quite 
sterile.  A  notable  example  of  this  is  the  cabbage-radish  hybrid, 
which  achieves  astounding  proportions,  but  is  completely  sterile 
(Gravatt  8). 

Here  the  loss  in  efficiency  in  the  reproductive  system  is  dis- 
tinctly not  accompanied  by  loss  in  efiiciency  in  vegetative  develop- 
ment. This  peculiarity  is  clarified  by  the  following  idea.  Wide 
crosses  involve  the  fusion  of  relatively  ''inharmonious"  gametes, 
w^hich  might  be  expected  to  produce  disturbances  in  the  ontogeny 
of  the  resulting  individual.  The  grosser  mechanism  which  regu- 
lates vegetative  development  can  evidently  weather  such  disturb- 
ances, while  the  more  finely  balanced  mechanism  of  gamete 
formation  is  upset. 


134  Outline  of  Genetics 

2.  Inbreeding. 

(This  again  might  be  used  as  an  example  of  opposite  extremes 
of  causes  producing  the  same  end  result,  for  inbreeding  can  surely 
be  regarded  as  the  antithesis  of  wide  crosses.) 

Inbreeding  commonly  results  in  "loss  of  vigor,"  through  the 
production  of  weakling  and  degenerate  plants  of  various  types 
(see  chapter  on  "Hybrid  vigor").  Frequently  these  degenerate 
types  exhibit  faulty  and  ineffective  reproductive  parts;  Types  of 
this  sort  have  frequently  been  obtained  through  inbreeding  corn. 

3.  Definite  hereditary  factors. 

A  good  example  of  this  appears  in  the  case  of  "tunicate"  or 
"podded"  corn.  Plants  homozygous  for  the  tunicate  factor  are 
sterile,  while  the  heterozygotes  are  partially  sterile  (Eyster  7). 

B.  Semi-sterility. 

1.  Wide  crosses. 

The  hybrids  produced  by  wide  crosses  are  not  in  all  cases 
completely  sterile  (see  III,  A,  i),  but  merely  show  an  abortion  of 
part  of  the  gametes,  notably  part  of  the  pollen.  This  phenomenon 
is,  in  fact,  of  such  general  occurrence  that  the  existence  of  a  certain 
amount  of  defective  pollen  is  frequently  used  as  a  criterion  of 
hybrid  origin.  ]VIany  plants  in  nature  have  been  found  to  show 
this  characteristic;  and  such  plants  have  been  called  "crypt- 
hybrids,"  the  impHcation  being  that  they  are  hybrids  that  have 
resulted  from  natural  crossing. 

2.  Inbreeding. 

Some  of  the  degenerate  plants  that  commonly  appear  as  a 
result  of  inbreeding  (see  III,  A,  2)  are  not  completely  sterile,  but 
merely  unsuccessful  in  setting  more  than  a  few  seeds.  This  may 
be  due  to  a  failure  of  part  of  the  pollen  or  part  of  the  ovules  or  part 
of  both. 

3.  Definite  hereditary  factors. 

Corn  which  is  heterozygous  for  the  tunicate  factor  (see  III, 
A,  3)  is  partly  sterile. 

Here  also  comes  a  very  unique  case,  which  will  be  described  in 
some  detail,  since  it  not  only  provides  an  ideal  example  of  sys- 
tematic semi-sterility  through  the  operation  of  definite  hereditary 
factors,  but  at  the  same  time  it  provides  an  example  of  inheritance 
in  the  gametophyte  generation. 


Sterility  135 

Bej.ling  (2)  made  a  cross  between  two  races  of  beans,  bolli  of 
which  were  completely  fertile.  'I'he  resulting  hybrids  were  semi- 
sterile.  Uniformly  just  one-half  of  the  pollen  grains  ai)fK'ared 
empty  and  collapsed,  while  one-half  of  the  ovules  had  no  embryo 
sacs.  The  sterile  pollen  and  ovules  appeared  in  random  distribu- 
tion with  the  fertile. 

Inbreeding  the  semi-sterile  hybrids,  Belling  obtained  an  F, 
generation  which  showed  the  following  features:  one  half  of  the 
plants  had  perfect  pollen;  the  other  half  had  a  mixture  of  equal 
numbers  of  good  and  bad  pollen  grains  in  all  their  flowers.  The 
plants  which  had  perfect  pollen  also  had  perfect  ovules,  while  the 
plants  with  50  per  cent  sterile  pollen  also  had  50  per  cent  sterile 
ovules.  In  the  F3  generation  all  the  descendants  from  the  fertile 
plants  had  perfectly  good  pollen  and  ovules;  but  the  progeny  of 
the  semi-sterile  plants  again  split  up  into  the  two  classes,  fertile 
and  semi-sterile,  as  before. 

Belling  states  his  general  conclusion  as  follows:  "The  e.x- 
planation  of  the  random  abortion  of  one-half  of  the  pollen 
and  one-half  of  the  embryo  sacs  must  apparently  be  by  the  segre- 
gation of  Mendelian  factors  among  pollen  grains  and  embr>'o 
sacs  individually,  and  not  by  the  action  of  these  factors  on  the 
zygotes." 

To  make  this  situation  clear  a  diagram  (fig.  21)  may  be  con- 
sidered. It  enlarges  a  little  upon  Belling's  original  ideas  as  he 
stated  them,  and  emphasizes  the  sporophyte-gametophyte  rela- 
tionship. Pollen  grains  and  embryo  sacs  are  gametophytes  in  the 
sense  that  they  include  the  male  and  female  gametophytes,  so  that 
W'hen  the  diagram  shows  sterile  gametophytes  it  is  the  same  as 
saying  that  both  pollen  grains  and  embr}'o  sacs  are  sterile.  This, 
of  course,  is  just  what  Belling  found;  whenever  one-half  the 
pollen  grains  in  random  distribution  were  sterile  one-half  the 
embryo  sacs  in  random  distribution  were  also  sterile. 

It  should  be  remembered  that  Belling  started  with  two  com- 
pletely fertile  races.  Suppose  that  the  parent  race  .  1  had  a  factor 
A'  whose  absence  brought  sterility  in  the  ganietophytes  (pollen 
grains  or  embryo  sacs).  Race  B  had  a  different  factor  1',  with 
similar  effect,  but  inheritetl  independently.  When  Helling 
crossed  these  races,  all  of  the  resulting  F,   hybrids  were  semi- 


136 


Outline  of  Genetics 


PARENTi 
RACE  ;  SPOROPHYTE: 
A 


®  0 


( 


0 


GAMET0PHYTES:1 


(FERTILE) 


(FERTILE) 


PARENT) 
RACE  f  SPOROPHYTE: 
B 


GAMETOPHYTES:/ 


(FERTILE) 


(FERTILE) 


F,  1  SPOROPHYTE; 


GAMETOPHHES: 


(FERTILE) 


(FERTILE) 


(STERILE) 


(STERILE) 


Fig.    21. — Diagram    illustrating   Belling's   explanation   of   semi- 
sterility. 


Stcrilltv 


137 


0 


0       0 


0 


0 


0       0 


sterile.  In  other  words,  in  every  1',  plant  one-hiilf  the  game- 
tophytes  were  sterile.  It  is  easy  to  see  why  XO  and  OY  arc 
fertile,  also  why  00  is  sterile  (lacking  both  factors);  but  why 
should  XY  be  sterile  when  it  has  both  factors  ?  Belling  explains 
it  by  saying  that  ganietophytes  are  unHkc  sporophytes  in  that 
they  normally  have  single  factors  instead  of  double  factors.  The 
germinal  capacity  of  a  gametophyte  is  just  one-half  that  of  a 
sporophyte.  It  is  as  if  a  gametophyte  were  "supersaturated" 
by  a  double  factor.  Such  a 
situation  is  abnormal  for  a 
gametophyte  and  brings  ab- 
normal results.  Therefore 
the  gametophyte  having  the 
abnormal  double  dose  {XY) 
is  just  as  sterile  as  the 
gametophyte  with  no  dose 
{00).^ 

In  developing  the  F2 
ratios  of  course  only  the  fer- 
tile gametophytes  function. 
XY  and  00  are  eliminated, 
so  far  as  posterity  is  con- 
cerned, so  that  we  have  to 
deal  only  with  the  chance 
matings  among  the  fertile 
gametes  {XO  and  OY).  According  to  the  laws  of  chance  there 
are  four  possible  matings  between  these  gametes  (fig.  22).  Out 
of  the  four  resulting  F2  sporophytes  two  would  evidently  pro- 
duce only  fertile  gametophytes  and  would  remain  fertile  as  long 

^  Belling's  rather  awkward  assumption  to  the  efTect  that  the 
gametes  with  the  double  dose  {XY)  are  non-functional  on  account  of 
being  "supersaturated"  might  be  improved  upon  by  the  following. 
Assume  race  A  has  complementary  factors  A'  and  O  for  fertility,  while 
race  B  has  a  similar  set  of  complementary  factors,  ()  and  Y,  located  on 
the  corresponding  chromosomes.  The  scheme  then  works  out  as  before, 
gametes  of  the  BO  and  AT'  formulas  both  being  non-functional  for  the 
same  reason  that  a  necessary  pair  of  comiilemcntary  factors  is  not 
present. 


0 


® 


Fig.  22. — Diagram  showing  how 
the  F2  would  be  produced  according 
to  Belling's  idea  of  semi-sterility. 


138  Outline  of  Genetics 

as  they  were  inbred.  The  other  two  are  exactly  Hke  the  original 
Fi  hybrid  and  therefore  semi-sterile,  having  one-half  sterile  gameto- 
phytes.     The  whole  dynasty  may  be  represented  as  follows: 


p 

F. 

Fertile  X  Fertile 
Semi -sterile 

F. 

50% 
Fertile 

Fertile 
Fertile 

50%  Semi-sterile 

F3 

50%  Fertile 

^^ 
Fertile 

50%  Semi-sterile 

1 

1 

F, 

50%  Fertile            50%  Semi-sterile 

This  is  a  very  ingenious  scheme,  and,  like  most  others,  should 
be  tested  by  further  experiments.  To  a  certain  extent  it  has  already 
met  this  test,  for  Belling  (3)  has  subsequently  reported  a  few 
more  generations  in  which  the  breeding  results  were  entirely  con- 
sistent with  those  of  the  earlier  generations.  Also  he  has  dis- 
covered two  new  races  of  beans  which  give  similar  results. 

C.  Self-sterihty. 

This  is  a  phenomenon  which  has  loomed  up  as  a  great  practical 
problem  during  the  last  few  years.  Where  pollen  and  ovules  are 
entirely  healthy,  and  functional  in  out  crosses,  but  quite  ineffective 
in  bringing  about  self-fertilization,  the  condition  of  self-sterility 
is  said  to  exist.  This  frequently  appears  in  certain  of  our  impor- 
tant horticultural  plants,  such  as  apples  and  plums,  so  that  it  has 
often  been  found  necessary  to  include  a  mixture  of  several  varieties 
within  the  orchard  in  order  to  insure  effective  pollination.  IVIany 
investigators  have  been  attacking  this  problem  with  various  tech- 
niques, the  most  critical  work  from  the  point  of  view  of  geneticists 
being  that  of  East  on  Nicotiana. 

By  crossing  self-sterile  with  self-fertile  species,  East  (6)  has 
demonstrated  that  self-sterility  behaves  in  inheritance  like  a 
simple  IVIendelian  recessive  to  self-fertility;  a  single  gene  evidently 
determines  the  difference  between  the  two  conditions.  East  (4) 
has  further  made  an  attempt  to  analyze  the  relations  of  self- 


Slcrilily  139 

sterile  plants  inler  se.  In  his  earlier  cultures  he  had  found  that 
the  self-sterile  plants  were  consistently  cross-fertile;  that  is,  there 
might  ])e  quite  a  group  of  individuals  each  one  of  which  would 
set  seed  when  pollinated  from  any  of  the  other  individuals,  but 
would  not  set  seed  when  self-pollinated.  East  concluded  that, 
when  one  is  deahng  with  self-sterile  plants  (those  lacking  the 
gene  for  self-fertility),  pollen  is  effective  only  when  it  comes  from 
a  source  that  has  a  somewhat  different  germinal  constitution  from 
that  of  the  ovules.  If,  therefore,  a  group  of  self-sterile  plants  is 
consistently  cross-fertile,  it  is  to  be  concluded  that  every  individual 
of  this  group  differs  in  some  degree  from  every  other  individual 
of  the  group  with  respect  to  a  certain  set  of  factors  that  is  effective 
in  this  connection.  If  this  assumption  is  correct,  it  should  be 
possible  in  the  later  generations  to  obtain  groups  of  individuals  all 
of  the  same  genotype  with  respect  to  the  effective  factors.  The 
individuals  of  any  such  group  should  then  be  cross-sterile  with 
reference  to  each  other.  East  actually  obtained  such  groups 
among  the  later  generations,  thus  supporting  his  assumptions  on 
the  relations  of  self-sterile  plants  inter  se.  An  exact  factorial 
analysis  has  not  been  possible  as  yet,  but  it  is  plainly  a  matter  of 
Mendelian  inheritance,  and  the  general  mechanism  is  rather 
clearly  indicated. 

Much  work  remains  to  be  done  on  the  physiology  of  self- 
sterility,  although  a  few  interesting  findings  have  already  been 
made  on  that  matter.  It  has  been  discovered  (at  least  for  a  great 
many  cases  of  self-sterility)  that  the  problem  is  tied  up  with  the 
growth  of  the  pollen  tube.  Own  pollen,  quite  healthy  and  func- 
tionable  on  foreign  stigmas,  will  also  germinate  and  start  pollen 
tubes  on  own  stigmas.  Such  tubes,  however,  are  for  some  reason 
not  successful  in  reaching  the  ovules.  Assumptions  were  made, 
by  various  authors,  that  own  stigmas  poison  own  pollen  tuljes 
or  furnish  them  with  inadequate  nutrition.  One  author  (Moore 
9)  has  assumed  that  own  stigmas  provide  own  pollen  tubes  with 
too  good  nutrition,  so  that  the  tubes  fatten  but  do  not  elongate 
(just  as  the  hypha  of  a  fungus  will  elongate  more  on  a  poor  nutri- 
tive medium  than  on  a  good  one).  East  (5)  himself  has  done  some 
critical  work,  however,  that  indicates  the  inaccuracy  of  all  of  the 
foregoing  assumptions,  and  reveals  an  interesting  phenomenon 


I40  Outline  of  Genetics 

that  actually  accounts  for  the  results  in  the  matter.  Through 
careful  sections  of  stigmas  made  at  intervals,  East  discovered  the 
following  fact.  Own  pollen  germinates  on  own  stigmas  just  as 
readily  as  does  foreign  pollen,  and  the  first  increment  of  growth  of 
the  two  types  of  tubes  takes  place  at  the  same  rate.  After  that 
own  pollen  tubes  continue  to  grow  steadily  and  "normally"  at 
the  same  rate  at  which  they  started,  but  the  rate  of  growth  of 
foreign  pollen  tubes  is  continuously  accelerated,  as  though  they 
were  receiving  some  stimulus  which  was  ineffective  on  own  pollen 
tubes.  The  result  is  that  own  pollen  tubes  fail  to  reach  the  ovary 
before  the  stigma  and  style  have  decayed,  while  foreign  pollen 
tubes,  with  their  accelerated  growth,  ''reach  the  goal  before  the 
road  has  become  blocked." 


LITERATURE  CITED 

1.  Allard,  H.  a.,  and  Garner,  W.  W.,  Flowering  and  fruiting 
of  plants  as  controlled  by  length  of  day.  U.S.  Dept.  Agric. 
Yearbook,  1920.  pp.  377-400. 

2.  Belling,  John,  K  study  of  semi-sterility.  Jour.  Heredity 
5:65-75.  1914. 

3.  ,    A    hypothesis    of    semi-sterility    confirmed.     Jour. 

Heredity  7:552.  1916. 

4.  East,  E.  M.,  and  Park,  J.  B.,  Studies  on  self-sterility.  I. 
The    behavior    of    self-sterile    plants.     Genetics    2:405-609. 

1917. 

5.  ,    Studies    on    self -sterility.     II.  Pollen    tube    growth. 

Genetics  3:353-366.^^5.  3.  1918. 

6.  East,  E.  ISI.,  Studies  on  self -sterility.  III.  The  relation 
between  self-fertile  and  self-sterile  plants.     Genetics  4:341- 

345-  1919- 

7.  Eyster,  W.  H.,  The  linkage  relations  between  the  factors  for 

tunicate  ear  and  sugary  endosperm  in  maize.     Genetics  6: 
209-240.  192 1. 

8.  Gravatt,  F.,  a  radish-cabbage  hybrid.  Jour.  Heredity  5: 
269-272.  1914. 

9.  Moore,  C.  W.,  Self-sterility.  Jour.  Heredity  8:203-207. 
1917. 


CHAPTER  XI 
THE  ENDOSPER]\I  IN  INHERITANXE 

We  have  dealt  chiefly  with  inheritance  in  the  sporo- 
phyte,  in  connection  with  which  most  of  the  work  in 
plant  genetics  has  been  done.  Brief  mention  has  been 
made  of  inheritance  in  the  gametophyte,  on  which  there 
has  been  very  little  work.  It  is  appropriate  now  to  con- 
sider inheritance  in  the  endosperm.  This  classihcation 
raises  the  question  as  to  the  nature  of  the  endosperm. 
It  was  at  one  time  generally  regarded  as  belonging  to  the 
gametophyte  generation,  but  since  the  discovery  of 
"double  fertilization"  in  1898  many  have  claimed  that  it 
belongs  to  the  sporophyte  generation.  On  the  basis 
of  chromosome  numbers,  it  is  neither,  so  that  there  is 
also  the  claim  that  endosperm  is  neither  sporophyte  nor 
gametophyte;  at  least  we  are  justified  in  considering 
inheritance  in  endosperm  as  a  separate  topic.  As  might 
be  inferred,  endosperm  shows  some  features  character- 
istic of  a  gametophyte,  others  characteristic  of  a  sporo- 
phyte, and  still  others  peculiar  to  itself.  Judgment  as  to 
its  nature,  therefore,  will  depend  on  which  of  these  fea- 
tures is  emphasized. 

It  is  generally  believed  that  angios})erms  have  been 
derived  from  gymnosperms,  and  it  is  natural  therefore 
to  explain  angiosperm  structures  by  the  corresj)onding 
structures  of  gymnos])erms.  11ie  gyninosj)erni  and  angio- 
sperm ovules  are  contrasted  in  fig.  2^,  whicli  will  assist 
in  the  following  discussion.     In  gymnosperms  the  situa- 

141 


142 


Outline  of  Genetics 


tion  is  clear.     After  the  germination  of  the  megaspore, 
everything  within  the  old  megaspore  wall  is  gametophyte 


Archegonium 

Egg 

Old  Megaspore     \  {        ^^  \__ \-    Fusion  Nucleus 

Wall,  Inclosing 

Female  Gametophyte  _ 

Old  Megaspore 

Wall,  Inclosing 

Female  Gametophyte 


Gymnosperm 


Angiosperm 


OVULE 


Gymnosperm 


Embrya 
Megaspore  Wall 

Endosperm 


SEED 


Angiosperm 


Embryo 
Megaspore  Wall 

Endosperm 


Fig.  23. — Diagram  contrasting  young  ovules  and  mature  seeds  of 
gymnosperms  and  angiosperms. 

tissue;  fertilization  affects  the  egg  only,  resulting  in  a 
sporophyte  embryo.  In  the  seed,  therefore,  the  embryo 
is  imbedded  in  nutritive  tissue  which  is  evidently  the 


The  Endosperm  in  InJicritance  143 

vegetative  body  of  the  female  gametophyte,  and  this  tis- 
sue is  called  the  endosperm.  In  angiospcrms,  however, 
a  new  situation  introduces  doubt.  It  can  be  said  as  before 
that  after  the  germination  of  the  megaspore  everything 
within  the  megaspore  wall  is  female  gametophyte  tissue, 
but  it  cannot  be  said  that  fertilization  affects  the  egg  only, 
for  one  of  the  sperms  fuses  as  regularly  with  the  fusion  nu- 
cleus as  does  the  other  sperm  with  the  egg.  It  will  be  re- 
membered that  the  fusion  nucleus  is  formed  by  two  nuclei, 
which  have  migrated  from  each  end  of  the  sac,  so  that 
when  the  sperm  enters  into  the  fusion  there  is  a  triple 
fusion.  After  fertilization  the  fertilized  egg,  of  course, 
forms  the  ombryo  sporophyte,  but  usually  every  nucleus 
of  the  old  gametophyte  disappears  except  the  fertilized 
fusion  nucleus,  which  then  forms  the  endosperm  in  which 
the  young  sporophyte  is  imbedded.  For  this  reason  the 
fertilized  fusion  nucleus  is  usually  called  the  endosperm 
nucleus. 

A  comparison  of  the  angiosperm  and  gymnosperm 
seeds  reveals  the  following  contrast  (fig,  23).  In  the 
appearance  of  their  essential  structures,  they  are  exactly 
alike,  and  on  that  basis  some  might  claim  that  the  endo- 
sperm of  angiosperms  is  the  same  as  that  of  gymnosperms, 
that  is,  gametophyte  tissue.  The  opposing  claim  is  that, 
although  the  gymnosperm  endosperm  is  gametophyte 
tissue,  the  situation  in  angiosperms  is  essentially  dif- 
ferent. In  angiosperms,  the  endosperm  docs  not  arise 
from  morphologically  unmodified  gameto})hyte  tissue, 
as  in  gymnosperms,  but  entirely  from  the  en(losj)erm 
nucleus,  and  this  nucleus  is  clearly  the  product  of  fusion 
of  male  and  female  nuclei.  With  such  an  origin,  the 
endosperm  nucleus  is  comparable  with  the  zygote,  and 


144  Outline  of  Genetics 

the  endosperm  tissue  is  sister  to  the  embryo  sporophyte. 
In  other  words,  in  angiosperms  the  endosperm  and 
embryo  are  twins.  This  means  that  the  endosperm  of 
angiosperms  belongs  to  the  sporophyte  generation, 
although  of  course  it  is  a  distinct  individual  which  pro- 
duces no  progeny.  The  embryo  sporophyte  is  a  parasite 
upon  its  twin  and  devours  it. 

It  will  be  recognized  that  there  is  some  reason  for 
both  of  these  claims.  Is  there  any  way  of  testing  the 
claims,  that  is,  of  distinguishing  between  sporophyte 
and  gametophyte  tissue  ?  The  cytological  distinction, 
based  on  chromosome  count,  is  that  the  sporophyte  is 
2X  tissue  and  the  gametophyte  is  x  tissue.  Applying  this 
test,  it  is  found  that  endosperm  tissue  is  neither  x  nor 
2X,  but  30;,  as  might  be  expected  from  the  triple  fusion. 
The  conclusion  involves  several  possibilities,  as  follows: 
30:  is  evidently  nearer  2X  than  x,  and  therefore  endosperm 
tissue  is  more  like  sporophyte  than  gametophyte  tissue; 
but  on  the  other  hand  two  of  the  x's  have  come  from  the 
female  gametophyte,  and  therefore  two-thirds  of  the 
endosperm  is  female  gametophyte.  On  the  basis  of 
predominance,  therefore,  endosperm  tissue  is  more  like 
female  gametophyte  tissue  than  anything  else.  Finally, 
there  is  a  third  alternative,  and  that  is  that  the  30;  con- 
dition deserves  to  be  set  apart  in  a  category  by  itself, 
which  would  mean  that  endosperm  is  neither  gameto- 
phyte nor  sporophyte. 

These  are  the  claims  and  the  evidence  as  to  the 
angiosperm  endosperm.  Opinion  is  not  settled,  but  the 
facts  are  clear.  This  prepares  for  a  consideration  of 
the  bearing  of  this  situation  upon  genetics.  The  geneti- 
cist is  not  much  concerned  about  the  exact  morphologi- 


The  Endosperm  in  InJieritance  145 

cal  or  physiological  nature  of  endosperm,  but  he  is  much 
concerned  about  its  behavior  in  inheritance.  J*erhaps 
the  phenomena  of  endosperm  inheritance  may  help  to 
decide  whether  endosperm  is  gametophyte  or  sporophy  te 
or  neither. 

Certain  races  of  corn  have  yellow  endosperm,  while 
in  other  races  it  is  white  (colorless).  If  a  cross  is  made 
with  pollen  from  the  yellow  endosperm  race  on  the  silks 
of  the  white  endosperm  race,  what  results  would  be 
expected  ?  We  could  assume  that  yellow  is  dominant 
over  white,  since  yellow  is  probably  due  to  the  presence 
of  a  factor  which  is  absent  in  white.  In  making  such  a 
cross,  therefore,  we  should  expect  a  hybrid  embryo  to  be 
formed  which  would  show  the  dominant  character  of 
yellow  endosperm  when  this  embryo  becomes  a  plant 
bearing  ears  the  next  season.  On  the  contrary,  we  tind 
the  dominant  yellow  character  appears  the  same  year 
that  the  cross  is  made.  The  cross,  of  course,  puts  the 
yellow  endosperm  factor  in  the  young  hybrid  embryo, 
but  we  cannot  imagine  that  this  embryo  passed  the 
character  out  into  the  endosperm  that  surrounds  it. 
The  real  mechanism  is  as  follows. 

Some  time  after  this  phenomenon  was  discovered  in 
1872,  it  was  named  xenia  (in  1881),  the  definition  of  the 
term  being  the  direct  effect  of  foreign  pollen  upon  the 
endosperm.  At  the  time  of  its  discovery  the  mechanism 
involved  in  xenia  was  not  understcK>d.  Later,  double 
fertilization  was  discovered,  and  this  furnished  the  neces- 
sary mechanism.  A  pollen  grain  from  the  yellow  endo- 
sperm race  contains  two  male  gametes,  and  each  gamete 
contains  the  factor  for  yellow  endosjHTin.  One  of  the 
gametes  fertilizes  the  egg  and  produces  a  hybrid  embryo, 


146  Outlme  of  Genetics 

which,  in  the  next  generation,  behaves  as  a  heterozygote 
for  yellow  endosperm.  The  other  male  gamete  fertilizes 
the  fusion  nucleus  and  produces  the  endosperm  nucleus, 
which  therefore  contains  the  factor  for  yellow,  the  result 
being  that  the  endosperm  is  yellow,  although  the  ovule 
belongs  to  the  white  race.  Xenia  means,  therefore,  that 
the  endosperm  is  a  hybrid  as  well  as  the  embryo,  and  the 
''triple  fusion"  involves  the  transmission  of  hereditary 
characters.  Fertilization  of  the  fusion  nucleus  is  just 
as  essential  as  fertilization  of  the  Qgg,  and  so  far  as  inherit- 
ance is  concerned  the  endosperm  and  embryo  are  sister- 
sporophytes. 

The  exact  function  of  double  fertilization  is  not 
clearly  understood.  Nemec  (7)  has  sought  to  account 
for  endosperm  hybridization  as  an  adaptation  which 
results  in  a  better  adjustment  of  the  composition  of  the 
reserve  food  supply  to  the  needs  of  a  hybrid  embryo. 

Xenia  throws  considerable  light  on  the  nature  of 
endosperm.  Because  of  its  behavior  in  inheritance, 
geneticists  would  naturally  regard  the  endosperm  as  a 
sporophyte,  an  abnormal  sister  to  the  embryo. 

The  phenomenon  of  xenia  is  not  limited  to  the  case 
of  yellow  endosperm,  but  appears  in  connection  with 
quite  a  number  of  endosperm  characters.  The  red- 
grain  and  purple-grain  characters  in  corn,  which  were 
employed  to  illustrate  types  of  factor  interaction,  are 
also  governed  by  this  mechanism.  In  these  cases,  how- 
ever, an  additional  detail  appears.  A  section  of  a  grain 
of  corn  appears  in  hg.  24.  There  is  first  the  pericarp  or 
"seed  coat,"  which  is  the  ovary  wall,  belonging  to  the 
old  sporophyte,  and  therefore  does  not  concern  us. 
Within  this  is  a  thin  alcurone  layer,  which  is  the  outer 


TJie  Endosperm  in  Tuheritance 


147 


layer  of  endosperm,  while  the  bulk  of  the  seed  consists 
of  the  starchy  endosperm.  Since  aleurone  is  endosj)erm, 
colors  peculiar  to  it  would  show  xenia  in  inheritance. 
This  was  the  case  in  East's 
red  and  purple  corn,  the 
colors  being  located  in  the 
aleurone  layer. 

There  is  another  phase 
of  the  situation  to  which 
attention  should  be  called. 
By  pollinating  the  silks  of  a 
white-grained  individual 
with  pollen  from  a  red- 
grained  individual,  xenia  is 
secured,  the  resulting  grains 


erm 


mbrvo 


Cross-Section  of 
Com-Seed 

Fig.  24. — Diagram  of  corn  seed 


being  red  like  those  of  the  pollen  parent.  In  the  reciprocal 
cross,  however,  that  is,  pollinating  silks  of  a  red-grained 
individual  with  pollen  from  a  white-grained  individual, 
a  different  result  is  obtained.  The  resulting  grains  are 
not  white  like  those  of  the  pollen  parent,  but  red  like 
those  of  the  ovule  parent.  There  is  no  xenia,  therefore, 
for  the  pollen  has  no  immediate  effect  upon  the  develop- 
ing endosperm.  This  seeming  difliculty,  however,  is 
easily  explained.  When  the  pollen  parent  is  white  and 
the  ovule  parent  is  red,  the  endosperm  gets  its  characters 
from  both  parents,  and  since  red  is  dominant  ()\'er  white 
the  resulting  endosperm  will  be  red  because  the  female 
nuclei  that  entered  into  the  triple  fusion  carried  the 
factor  for  red  endosperm;  and  therefore  the  pollen  from 
the  white  ])arent  seemed  to  ha\c  no  cIUh  t.  'i1u'  mecha- 
nism works  in  all  cases,  but,  owing  to  dominance,  xem'a 
appears  only  in  certain  cases.     'Jliere  is  no  need  to  dis- 


148  Outline  of  Genetics 

cuss  all  of  the  Mendelian  situations  in  which  xenia  may 
occur.  An  understanding  of  the  underlying  mechanism 
should  enable  us  to  analyze  such  cases  and  reach  a  con- 
clusion as  to  the  expected  results. 

A  law  which  East  (i)  has  formulated  in  reference  to 
xenia  is  pertinent:  ''When  two  races  differ  in  a  single 
visible  endosperm  character,  in  which  dominance  is  com- 
plete, xenia  occurs  only  when  the  dominant  parent  is 
male  (pollen  parent).  When  the  two  races  differ  in  a 
single  endosperm  character,  in  which  dominance  is  incom- 
plete, or  when  they  differ  in  two  characters  (factors), 
both  of  which  are  necessary  for  the  development  of  the 
visible  difference,  in  both  of  these  cases  xenia  occurs 
when  either  parent  is  male."  This  may  be  called  the 
law  of  ''normal"  xenia.  What  may  be  called  "abnor- 
mal" xenia  should  now  be  considered. 

In  connection  with  some  of  his  work  on  sweet  and 
starchy  corn,  East  (i)  was  able  to  distinguish  two  distinct 
races  of  starchy  corn.  In  one  race  the  starch  occurred 
in  a  loose  powdery  or  floury  condition,  while  in  the 
other  race  it  was  compacted  into  a  hard,  flinty,  or  so-called 
corneous  condition.  The  two  races,  therefore,  may  be 
spoken  of  as  floury  and  corneous  races  of  starchy  corn. 

East  made  various  crosses  between  these  two  races 
to  discover  the  method  of  inheritance  of  the  two  endo- 
sperm characters.  Naturally  such  characters  would  be 
expected  to  show  xenia.  In  the  following  description, 
therefore,  when  the  Fi  generation  is  referred  to,  both  the 
hybrid  embryo  and  the  hybrid  endosperm  surrounding 
it  will  be  included. 

When  East  used  the  floury  race  as  the  pollen  parent 
and  the  corneous  race  as  the  ovule  parent,  the  Fi  genera- 


Tlie  Kudos  perm  iji  TfiJfcritance  149 

tion  was  all  corneous.  When  he  made  the  reciprocal 
cross  (corneous  pollen  and  lloury  ovule),  the  Fj  genera- 
tion was  all  floury.  This  result  certainly  suggests 
maternal  inheritance,  for  in  both  cases  it  is  the  character 
of  the  ovule  parent  that  is  transmitted.  If  it  is  assumed 
that  this  is  a  case  of  maternal  inheritance,  two  problems 
are  encountered:  (i)  to  prove  that  this  behavior  is  not 
due  merely  to  parthenogenesis;  (2)  to  discover  the 
mechanism  to  explain  maternal  inheritance  in  this  case. 
In  the  first  place,  East  established  the  fact  that  there 
was  no  possibility  of  parthenogenesis.  Continuing  his 
investigation,  he  inbred  the  Fi  generation  in  each  case 
and  examined  the  F2  progeny.  If  he  were  dealing  with 
a  case  of  maternal  inheritance,  what  should  the  F2  genera- 
tion show?  It  should  be  exactly  the  same  as  the  Fi 
generation,  for  in  true  maternal  inheritance  a  race  will 
go  on  forever  breeding  true  to  the  maternal  character, 
whether  it  is  self -pollinated  or  cross-pollinated.  If  this 
had  been  a  case  of  true  maternal  inheritance,  East 
should  have  obtained  the  following  results: 

Floury  X  Corneous  Corneous         X  Floury 

(ovule  parent)      n|/  (ovule  parent)      i' 

Fi  Floury  Corneous 

Fa  Floury  Corneous 

etc.  etc. 

Actually,  however,  he  obtained  the  following  results : 

Floury  X  Corneous  Corneous         X  Floury 

(ovule  parent)      ^  (ovule  parent)      \1^ 

Fi  Floury  Corneous 


^  Floury  :  I  Corneous  h  Corneous  :  ^  Floury 


150  Outline  of  Genetics 

The  conclusion  is  that  this  is  not  a  true  case  of  mater- 
nal inheritance.  East  offers  a  very  reasonable  explana- 
tion of  these  results,  based  upon  the  peculiarities  of 
double  fertilization.  These  characters  appear  super- 
ficially to  be  maternal  for  the  following  reasons. 
The  endosperm  nuclei  are  30:,  2x  from  the  female  and  x 
from  the  male.  In  the  characters  under  discussion,  the 
presence  of  two  factors  always  dominates  the  presence 
of  one  factor;  thus  corneous  female  crossed  with  floury 
male  produces  progeny  that  are  all  phenotypically  corne- 
ous, while  floury  female  crossed  with  corneous  male  for 
the  same  reason  produces  progeny  which  are  all  pheno- 
typically floury.  The  mother  always  determines  the 
character  of  the  hybrid  endosperm  because  there  are 
always  two  female  nuclei  to  predominate  over  the  single 
male  nucleus.  In  the  embryo,  however,  this  predomi- 
nance does  not  occur,  for  there  only  a  single  female 
nucleus  has  fused  with  the  single  male  nucleus.  When 
this  hybrid  embryo  matures,  therefore,  it  is  evident  that 
it  will  produce  gametes  of  two  sorts,  50  per  cent  corneous 
and  50  per  cent  floury.  Since  the  female  is  really  the 
only  decisive  factor,  so  far  as  endosperm  is  concerned,  the 
ratios  appearing  among  the  female  gametes  in  the  Fj 
generation  will  be  the  ratios  that  will  appear  also  in  the 
F2  endosperms.  In  other  words,  50  per  cent  of  the  F, 
endosperms  will  be  corneous  and  50  per  cent  floury,  no 
matter  what  may  be  the  source  of  the  pollen.  It  is 
obvious  that  the  explanation  of  this  peculiar  form  of 
apparently  maternal  inheritance  depends  entirely  upon 
a  clear  conception  of  the  phenomenon  of  triple  fusion. 
Conversely,  this  type  of  inheritance  indicates  that  the 
triple   fusion,    instead   of   being  merely   a   meaningless 


TJie  Eiidosper))!  in   TjiJicrUance  151 

cytological   peculiarity,   is  really   significant   in    inherit- 
ance. 

The  foregoing  case,  which  is  the  only  endosperm 
character  that  is  inherited  in  this  peculiar  way,  provides 
a  conspicuous  criticism  of  the  presence  and  absence 
hypothesis  (see  also  pp.  51,  112).  Whichever  of  the  two 
characters  be  assumed  to  be  due  to  the  presence  of 
a  gene,  a  demonstration  is  provided  to  the  effect  that 
two  absences  will  dominate  one  presence.  In  spite 
of  this  outstanding  exception,  the  terminology  of  the 
presence  and  absence  hypothesis  is  retained  by  practi- 
cally all  geneticists,  and  is,  in  fact,  employed  even  in 
connection  wath  the  case  of  corneous  and  floury  endo- 
sperm. 

Webber  (8),  in  1900,  experimenting  on  xenia  in  corn,  uncov- 
ered some  interesting  anomalies.  Pollen  from  a  red-grained  race, 
applied  to  silks  of  a  white-grained  race,  should  result  in  solid  red 
grains  if  xenia  is  normal.  Of  course  Webber  actually  obtained 
this  result  in  the  vast  majority  of  cases,  but  occasionally  there 
appeared  two  other  types  of  grains :  (a)  white  grains  covered  with 
numerous,  irregular  patches  of  red,  commonly  called  ''mottled"; 
{b)  grains  of  which  a  large  and  uninterrupted  area  of  the  aleurone 
was  pure  white,  while  the  remaining  area  of  the  aleurone  was  just 
as  pure  red,  commonly  called  "mosaic."  For  these  cases  he  con- 
structed an  ingenious  explanation. 

Normally,  the  second  male  nucleus  fuses  with  the  fusion 
nucleus,  and  the  result  is  a  solid  red  grain.  In  some  cases,  how- 
ever, the  second  male  nucleus  (i)  does  not  join  with  the  other  two; 
while  in  still  other  cases  the  second  male  nucleus  (2)  fuses  with  but 
one  of  the  polar  nuclei,  leaving  the  other  polar  nucleus  to  act 
independently.  Either  of  these  irregularities,  Webber  felt,  would 
serve  to  account  for  the  anomalous  grains,  for  in  either  case  female 
nuclei  would  be  left  to  act  independently  in  the  formation  of  part 
of  the  endosperm.  That  part  of  the  endosperm  would  neces- 
sarily show  colorless  aleurone,  since  the  female  parent  to  the  cross 


152  Outline  of  Genetics 

could  contribute  no  color.  The  remainder  of  the  endosperm, 
produced  by  the  male  nucleus  (either  (i)  independently  or  (2) 
in  conjunction  with  one  of  thepolars),  would,  of  course,  show  the 
red  aleurone  characteristic  of  the  male  parent. 

The  arrangement  of  the  red  and  white  areas,  sometimes  accord- 
ing to  the  mottling  plan  and  sometimes  according  to  the  mosaic, 
might  be  explained  by  the  usual  method  of  endosperm  formation. 
Endosperm  formation  begins  with  free  nuclear  division,  the  result- 
ing nuclei  being  free  in  the  cytoplasm  of  the  embryo  sac.  The 
cell  walls  are  not  formed  for  some  time;  sometimes  not  until 
nuclear  division  is  completed.  Before  a  large  number  of  free 
nuclei  have  appeared  they  move  from  the  central  region  of  the  sac 
and  usually  become  placed  near  the  wall,  where  free  nuclear  divi- 
sion continues.  When  walls  begin  to  appear,  separating  the 
nuclei,  wall  formation  begins  at  the  periphery  of  the  sac  and 
extends  toward  the  center,  in  what  is  called  centripetal  growth. 
This  program,  which  is  common  in  seed  plants  and  is  known  to 
occur  in  wheat,  is  doubtless  the  program  in  corn.  If,  then,  the 
second  male  nucleus  fails  to  unite  with  the  fusion  nucleus  and  each 
divides  separately,  when  their  progeny  nuclei  move  out  to  the 
periphery  of  the  sac  the  nuclei  of  male  and  female  origin  may 
well  become  more  or  less  mixed.  In  their  further  division,  there 
w^ould  be  groups  of  cells  of  male  origin  interspersed  among  groups 
of  female  origin.  The  result  would  be  red  and  white  areas  on 
the  mature  grain,  intermingled  as  irregular  blotches,  giving  the 
mottled  effect  (a).  On  the  other  hand,  if  the  daughter-nuclei  of 
the  male  and  female  components  migrated  en  bloc  to  the  wall  of 
the  embryo  sac,  and  no  mixing  occurred  between  nuclei  of  the 
tw^o  types,  the  result  would  be  the  production  of  anomalous  grains 
of  the  mosaic  type  {b). 

These  ingenious  proposals  of  Webber's  helped  to  focus  the 
attention  of  other  investigators  upon  the  problem  imposed  by  the 
occurrence  of  anomalous  grains  of  these  two  types. 

Webber's  conception  of  the  mottled  grains  (a)  was  shown 
to  be  fallacious  by  the  experiments  of  Kempton  (6)  and  of  Emer- 
son (4).  It  was  found  that  mottled  grains,  instead  of  being 
anomalies  as  Webber  had  believed,  would  appear  in  considerable 
numbers  and  with  dependable  regularity  under  the  proper  con- 


The  Endosperm  in  Inheritance  153 

ditions.  The  required  conditions  were  peculiar  and  interesting. 
If  the  R  factor  enters  the  cross  with  the  male  parent  only,  a  mottled 
aleurone  results;  if  the  R  factor  enters  with  the  female  parent  only 
or  with  both  parents,  solid  red  is  the  result.  Thus  there  is  a  pheno- 
typic  distinction  between  grains  having  but  one  dose  of  the  R 
factor  (male  parent  only)  and  those  having  two  (female  parent 
only)  or  three  doses  (both  parents).  Furthermore,  this  phenome- 
non will  not  appear  in  all  races  of  corn,  but  only  in  those  which 
contain  a  dominant  factor  for  mottling  {S,  for  "spotted"  aleurone, 
as  Kempton  calls  it). 

Webber's  explanations,  (i)  and  (2),  however,  might  still 
apply  to  the  anomalous  grains  of  the  mosaic  type  (6).  Proposi- 
tion (i),  that  the  second  male  nucleus  fails  to  fuse,  and  acts  inde- 
pendently in  endosperm  formation,  was  proved  to  be  impossible 
by  some  of  the  experiments  of  East  (2).  Factors  R  and  C  must 
be  present  simultaneously  for  the  production  of  red  aleurone.  A 
cross  between  two  colorless  types,  CCrr  and  ccRR,  therefore, 
should  produce  only  red  grains.  Even  here,  however,  aberrant 
grains  sometimes  appear,  part  of  the  grain  being  red  and  the  rest 
colorless.  Failure  of  the  second  male  nucleus  to  fuse  with  the 
female  polar  nucleus  in  such  a  case  would  result  in  a  grain  which 
was  entirely  colorless,  a  thing  which  never  occurred.  It  is  only 
by  fusion  of  male  and  female  nuclei  that  any  part  of  the 
aleurone  can  be  red.  The  experiments  on  this  point  were  suf- 
ficiently extensive  to  demonstrate  that  the  second  male  nucleus 
never  fails  to  affect  a  fusion  with  at  least  one  of  the  female 
nuclei. 

There  yet  remained,  however,  Webber's  possibility  (2), 
fusion  of  the  second  male  nucleus  with  only  one  of  the  female 
polars,  the  other  female  polar  acting  independently.  This  last 
possibility  was  disproved  by  Emerson'  (3)  in  the  following  inter- 
esting manner.  A  colorless,  sugar>'  type,  CCrrsusu,  was  used  as 
female  parent  in  a  cross  with  a  colorless,  starchy  type,  ccRR^uSu. 
The  resulting  grains  were  red,  starchy,  save  for  a  few  aberrant 
grains  which  were  red  in  part  and  colorless  in  part,  but  starchy 
throughout.  Webber's  proposition  (2)  fails  here,  since  fusion 
of  the  second  male  nucleus  with  only  one  of  the  polars  would  pro- 
duce grains  which  were  red,  starchy  in  part  (from  male  nucleus 


154  Outline  of  Genetics 

fused  with  one  polar)  and  colorless,  sugary  in  part  (from  independ- 
ent polar). 

These  critical  experiments  served  to  disprove  Webber's  prop- 
ositions and  proved  that  the  normal  program  of  double  fertiliza- 
tion is  invariable  in  corn.  The  occurrence  of  the  occasional 
anomalous  mosaic  grains,  however,  remained  to  be  explained. 
"Somatic  mutation"  was  invoked  by  some  as  an  explanation,  but 
proved  unsatisfactory  for  a  number  of  reasons. 

Emerson  (5)  has  finally  obtained  critical  evidence  which 
indicates  a  very  satisfactory  explanation  of  the  phenomenon. 
The  factor  wx  for  waxy  endosperm  {Wx,  corneous  endosperm) 
is  known  to  be  carried  on  the  same  chromosome  with  the  C  factor. 
A  cross  was  made  between  a  colorless,  waxy  female  parent,  c-wx 
c-wx,  and  a  red  corneous  male  parent,  C-Wx  C-Wx  (the  R  factor 
being  present  in  both  parents).  The  resulting  triploid  endosperm 
was  of  the  formula  c-wx  c-wx  C-Wx.  If  non-disjunction  (passing 
of  both  halves  of  a  divided  chromosome  to  one  pole)  occurred  in 
connection  with  the  third  of  these  chromsomes,  one  of  the  result- 
ing nuclei  would  be  diploid  for  this  chromosome  set,  c-wx  c-wx, 
and  the  other  tetraploid,  c-wx  c-wx  C-Wx  C-Wx.  Endosperm 
produced  by  the  former  should  be  colorless,  waxy;  endosperm 
produced  by  the  latter  should  be  red,  corneous.  Emerson 
obtained  aberrant  grains  which  were  of  exactly  this  constitution, 
the  colorless  areas  being  at  the  same  time  waxy  and  the  red  areas 
corneous.  This  experiment,  considered  together  with  the  pre- 
vious ones,  indicates  that  occasional  non-disjunction  is  the  expla- 
nation of  these  aberrant  grains. 

(The  frequency  of  these  particular  aberrant  grains  is  one  in 
423,  and  one  may  expect  non-disjunction  to  take  place  in  connec- 
tion with  some  one  chromosome  in  the  corn  endosperm  in  about 
one  of  every  fourteen  grains.  Direct  cytological  demonstration  is 
to  be  hoped  for.  Non-disjunction  is  known  to  occur  at  times 
elsewhere  in  the  plant  and  animal  kingdoms.  Possibly  the  trip- 
loid nature  of  endosperm  furnishes  an  especially  favorable  condi- 
tion for  its  occurrence.) 

This  fascinating  series  of  experiments  shows  how  features  of 
the  morphological  and  cytological  program  in  a  plant  may  be 
demonstrated  in  a  very  convincing  way  through  the  indirect  evi- 


TJic  Endosperm  in  TnJicrilance  155 

dcncc  provided  by  c.-ircful  breeding;  cxi)criincnts.  where  it  would  be 
rather  hopeless  to  effect  any  sueh  coin^ineing  demonstration 
through  direct  morphological  or  i\-(ologi(al  examination. 

LITERATURE  CITED 

1.  East,  E.  M.,  and  Hayhs,  H.  K.  Inheritance  in  maize.  Conn. 
Agric.  Exper.  Sta.  Bull.  no.  167.  pp.  142.  pis.  25.  191 1. 

2.  East,  E.  M.,  Xenia  and  the  endosperm  of  angiospcrms.  Bot. 
Gaz.    56:217-224.  1913. 

3.  Emerson,  R.  A.,  Anomalous  endosperm  development  and  the 
phenomenon  of  bud  sports.  Zeit.  Induk.  Abstamm.  \'ererb. 
14:241-259.  1915. 

4. — ,  A  fifth  pair  of  factors,  Aa,  for  aleurone  color  in  maize, 

and  its  relation  to  the  Cc  and  Rr  pairs.  Cornell  Univ.  Agric. 
Exp.  Sta.  Mem.  16.  pp.  231-289.  1918. 

5. ,  Genetic  evidence  of  aberrant  chromosome  behavior  in 

maize  endosperm.     Amer.  Jour.  Bot.  8:411-424.  fig.  i.  192 1, 

6.  Kempton,  J.  H.,  Inheritance  of  spotted  aleurone  color  in 
hybrids  of  Chinese  maize.     Genetics  4:261-274.  Jigs.  3.   1919. 

7.  Nemec,  B.,  Das  Problem  der  Befruchtungsvorgange.  Berlin. 
1910. 

8.  Webber,  H.  J.,  Xenia,  or  the  immediate  effect  of  pollen  in 
maize.     U.S.  Dept.  Agric.  Bull.  no.  22.  pp.  44.  pis.  4.  i9CX3. 


CHAPTER  XII 
HYBRID  VIGOR 

The  phenomenon  of  hybrid  vigor  has  already  been 
referred  to.  It  is  a  matter  so  intimately  related  to 
genetics,  particularly  plant  genetics,  both  on  the  theo- 
retical side  and  in  connection  with  practical  breeding, 
that  it  will  be  worth  while  to  consider  it  in  some  detail  here. 

The  first  record  of  observations  on  hybrid  vigor  is  that 
of  KoLREUTER  in  1776,  who  states  that  crossing  results 
in  an  increase  of  general  vegetative  luxuriance  and  in 
an  increase  in  the  facility  of  vegetative  propagation  and 
viability.  Later  Gartner  discussed  the  same  phenome- 
non but  gave  no  important  new  ideas.  Finally,  hybrid 
vigor  attracted  the  attention  of  Darwin  (4),  who  states 
that  crossing  hastens  the  time  of  flowering  and  maturing 
and  increases  the  size  of  the  individual.  He  adds  the 
very  important  fact  that  it  is  not  mere  crossing  that 
gives  the  stimulus,  but  crossing  forms  that  differ  in  the 
constitution  of  their  sex  elements ;  in  other  words,  cross- 
ing between  different  flowers  on  the  same  plant  gives 
no  advantage,  nor  does  crossing  two  individuals  which 
are  gemiinally  identical.  He  assumed  (incorrectly,  see 
p.  161)  that  any  effective  germinal  dift'erence  was  to  be 
accounted  for  by  the  fact  that  the  parents  had  been 
growing  under  different  environmental  conditions.^ 

^  It  is  probably  Darwin  who  is  responsible  for  bringing  hybrid 
vigor  to  the  attention  of  botanists,  although  the  modern  popular  impres- 
sion might  be  that  Burbank  deserves  the  credit  because  of  his  experience 
in  producing  some  remarkably  fast-growing,  large,  and  vigorous  hybrids. 

156 


Hybrid  Vigor  i^y 

Even  ]\Iendel's  classic  pea  hybrids  supplic-d  furllier 
instances  of  increase  in  size  resulting  from  crossing. 
''Stems  of  I  foot  and  6  feet  in  length  yielded  without 
exception  hybrids  which  varied  in  length  between  6  feet 
and  7I  feet"  (see  East  and  Jones  6). 

Among  the  modern  investigators  of  hybrid  vigor. 
Shull,  East,  and  Jones  have  contributed  much  toward 
an  explanation  of  the  phenomenon. 

Skull's  (12)  conclusions  up  to  the  year  igio  may  be 
summarized  as  follows.  His  work  was  entirely  with 
corn,  and  the  conclusions  contained  some  very  significant 
points. 

1.  "The  progeny  of  every  self-fertilized  corn  plant 
is  of  inferior  size,  vigor,  and  productiveness,  as  com- 
pared with  the  progeny  of  a  normally  cross-bred  plant 
derived  from  the  same  source."  In  general  this  con- 
clusion would  be  admitted  by  everyone,  but  it  raised 
one  question.  It  was  known  that  when  two  races  have 
been  inbred  for  many  generations  they  frequently  "run 
out,''  gradually  losing  their  vigor.  In  such  a  case  a  cross 
between  the  two  races  tends  to  restore  the  original  vigor. 
The  remaining  question,  however,  is  whether  the  same 
result  may  be  effected  by  a  cross  between  two  inbred 
races  which  have  not  run  out,  but  remain  in  normal  \igor. 
Shull  answers  that  hvbrid  vigor  is  exhibited  when  both 
parents  are  above  the  average  condition  as  well  as  when 
they  are  below  it. 

2.  Another  question  which  naturally  ari.^es  is  as 
follows.  When  these  crosses  are  made  it  is  of  course  the 
Fx  generation  that  shows  the  hybrid  vigor.  If  the 
Fi  generation  is  inbred,  what  is  the  status  of  the  Fj 
and  later  generations  with  reference  to  vigor?     SiiULL 


158 


Outline  of  Genetics 


answers  this  question  in  the  following  general  way. 
''The  decrease  in  size  and  vigor  which  accompanies  self- 
fertilization  is  greatest  in  the  first  generation  and  becomes 
less  and  less  in  each  succeeding  generation,  until  a  con- 
dition is  reached  in  which  there  is  (presumably)  no  more 
loss  of  vigor."  The  facts  involved  in  this  statement 
may  be  represented  in  fig.  25.  In  this  figure,  it  can  be 
seen  clearly  that  the  great  loss  of  vigor  comes  immediately 
after  self-fertilization  again  begins.  After  that,  self- 
fertilization  brings  additional  loss  in  vigor,  but  this  loss 


Parent 
Races 


F, 


F, 


F,   Fg    etc. 


Fig.  25. — Illustrating  status  of  hybrid  vigor  in  Fi  and  later  genera- 
tions.    Vigor  represented  by  height  of  rectangles. 

is  less  with  each  succeeding  generation.  It  is  as  though 
a  very  definite  limit  were  being  approached  and  each 
generation  goes  down  one-half  of  the  remaining  distance 
toward  that  Hmit.  Just  why  and  in  what  way  this  limit 
is  approached  will  be  considered  later  in  connection  with 
the  work  of  East  and  Jones. 

3.  ''A  cross  between  sibs  (sister  and  brother)  within 
a  self-fertilized  family  shows  little  or  no  improvement 
over  self-fertilization  in  the  same  family."  This,  it 
will  be  noticed,  is  simply  carrying  a  little  further  the 
point  that  Darwin  originally  discovered.     We  realize 


Hybrid  Vi^or  159 

that  an  inbred  race  should  be  homozyjjjous;  therefore  all 
the  indi\'i(luals  involved  would  ha\e  the  same  f^erminal 
constitution.  A  cross  between  any  two  such  individuals 
would  really  not  be  effective  in  producing  a  hybrid,  so 
that  it  would  not  be  surprising  that  such  a  cross  fails  to 
bring  hybrid  vigor. 

4.  ''A  cross  between  plants  belonging  to  two  self- 
fertilized  families  results  in  a  progeny  of  as  great  vigor, 
size,  and  productiveness  as  are  possessed  by  families 
that  have  never  been  self-fertilized."  The  conclusion 
from  this  is  that  inbreeding  results  in  no  permanent 
loss  of  vigor.  A  race  may  "run  out''  if  inbred  continu- 
ously, but  when  crossed  with  another  race  it  immediately 
seems  to  regain  all  the  original  vigor.  It  is  as  though  all 
germ  plasm  contains  the  potentiality  of  developing  vig- 
orous individuals.  This  potentiality,  however,  cannot 
express  itself  until  the  proper  combination  of  conditions 
arises,  and  this  proper  combination  seems  to  be  connected 
in  some  way  with  hybridizing. 

5.  ''Reciprocal  crosses  between  two  distinct  self- 
fertilized  families  are  equal"  in  producing  hybrid  vigor. 
When  reciprocal  crosses  are  equal  it  suggests  a  Mendclian 
phenomenon.  Is  it  possible  that  hybrid  vigor  may  be 
explained  in  terms  of  Mendelism  ? 

These  are  five  "laws"  of  hybrid  vigor  presented  by 
Shull,  in  1910.  It  should  be  noted  that  they  are  not 
hypotheses  but  observed  facts.  The  hypotheses  were 
developed  later  when  more  of  the  facts  were  in. 

A  practical  suggestion  made  by  Siiui.i.  in  connection  with 
hybrid  vigor  is  of  interest.  Ciranted  that  hybrid  vigor  is  an  estab- 
Ushcd  fact,  the  question  of  its  practical  use  in  connection  with 
crop  plants  should  be  taken  into  account.     If  a  farmer  after  years 


i6o 


Outline  of  Genetics 


of  work  has  finally  developed  a  desirable  new  strain  of  corn  by 
selection,  he  is  not  likely  to  favor  hybridizing  with  some  other 
strain  in  any  wholesale  way.  He  must  preserve  his  pure  strain 
at  all  costs.  Shull  has  suggested  the  following  solution  of  this 
practical  problem,  as  indicated  in  fig.  26.  Two  desirable  strains 
{A  and  B)  are  developed.  One  small  plot  (I)  is  planted  entirely 
with  A,  and  at  some  distance  another  small  plot  (II)  is  planted 
with  A  and  B  in  alternating  rows.  Plot  I  is  used  only  to  perpetu- 
ate A  in  pure  condition.  In  plot  II  all  the  .1  plants  are  detasseled. 
The  silks  of  these  .1  plants,  therefore,  are  pollinated  by  B  pollen 
only,  and  the  resulting  grains  in  the  .1  rows  are  all  bound  to  be 
hybrids.     Using  these  grains  as  seed  for  the  crop,  hybrid  vigor 

A      B      A     B      A     B 


PLOT  I  PLOT  11 

Fig.  26. — Shull's  scheme  of  planting  for  making  practical  use  of 
hybrid  vigor  in  corn. 


will  be  obtained.  At  the  same  time  both  .4  and  B  are  perpetuated 
in  the  pure  condition,  since  the  B  rows  in  plot  II  are  always  self- 
pollinated,  as  there  is  no  other  pollen  in  that  neighborhood.  This 
is  a  very  simple  solution  of  the  problem,  without  necessitating 
laborious  hand  pollination. 

The  investigations  and  conclusions  of  East  (5)  may 
next  be  considered.  Shull  did  his  work  entirely  with 
corn,  but  East  investigated  the  problem  in  a  more  whole- 
sale way.  After  assembling  an  extensive  collection  of 
data,  he  made  the  summarizing  statement  that  59  out  of 
85  angiosperm  crosses  showed  a  noticeable  increase  in 


Hybrid  Vii^or  i6i 

vigor.  East  of  course  did  not  conliniR'  to  investigate 
all  of  these  85  types,  but  concentrated  ui)()n  two  repre- 
sentatives. Corn  was  selected  as  represent in<(  s[)ecies 
normally  cross-fertilized  in  nature,  while  tobacco  was 
used  to  represent  those  species  generally  self-fertih'zed 
in  nature. 

East's  results  with  corn  need  not  be  discussed  in 
detail,  for  they  confirmed  Shull's  results  in  every  point. 
It  was  found  that  crosses  between  plants  of  aj)pi"()xi- 
mately  the  same  genotype  resulted  in  little  or  no  hybrid 
vigor,  even  in  cases  where  the  two  parents  to  the  cross 
had  been  grown  under  different  environmental  conditions 
(thus  correcting  Darwin's  misconception,  see  p.  156). 
It  was  also  observed  that  some  crosses  resulted  in  rela- 
tively less  hybrid  vigor  than  others.  From  such  results 
East  developed  a  very  significant  and  useful  Mendelian 
interpretation  of  hybrid  vigor.  His  proposition  is  that 
hybrid  vigor  is  proportional  to  the  number  of  factors  in 
which  the  parents  differ.  This  situation  may  be  \'isu- 
alized  from  the  following  diagram. 

Parents  !•  i 

AABBCCDDXAABBCCdd  =  AABBCCDd  =  \iii\c  hybrid  vigor 
AABBCCDDX  AABBccdd  =  AABBCcDd  =  more  hybrid  vigor 
AABBCCDDX  AAbbccdd   =  .1.1  BhCcDd  =  still  more  hybr  id  vigor 
AABBCCDDX  aabhccdd      =    AaBbCcDd  =  mo<.i  hybrid  vigor 

It  is  the  Fi  of  course  that  shows  the  vigor,  but  what 
index  can  be  obtained  from  the  germinal  formula  of  the 
Fi  generation  as  to  the  amount  of  hybrid  \  igi)r  that  it 
will  show?  It  is  evident  that  this  index  lies  in  the  fact 
that  hybrid  vigor  is  proportional  to  the  innnber  of  factors 


1 62  Outline  of  Genetics 

that  are  in  the  heterozygous  condition  in  the  Fi  genera- 
tion. Thus  in  the  first  case  shown  in  the  diagram  there 
is  only  a  single  heterozygous  set  {Dd),  and  the  result  is 
little  hybrid  vigor.  Following  down  the  diagram  it 
will  be  noted  that  2,3,  and  4  of  these  heterozygous  sets 
bring  an  increasing  amount  of  hybrid  vigor.  These  are 
the  facts  that  lie  at  the  basis  of  East's  theory  which  he 
calls  heterozygosis.  This  term  should  not  be  confused 
with  heterosis,  which  is  commonly  used  to  express  merely 
the  fact  of  hybrid  vigor. 

We  shall  now  consider  how  this  conception  of  hetero- 
zygosis serves  to  account  for  the  phenomena  that  Shull 
had  previously  discovered  in  connection  with  hybrid 
vigor. 

1.  The  fact  of  hybrid  vigor. — Heterozygosis  suggests 
that  hybrids  are  vigorous  on  account  of  the  heterozygous 
sets  of  factors  that  they  contain. 

2.  The  decrease  in  vigor  after  self-fertilization  begins 
again.^ — The  greatest  loss  in  vigor  comes  between  the  Fj 
and  F2  generations.  Thereafter  the  loss  becomes  gradu- 
ally less  each  generation,  approaching  a  definite  limit 
beyond  which  no  further  loss  in  vigor  occurs.  Heterozy- 
gosis explains  this  as  follows: 

AABBCCDDXaabhccdd  =  AaBhCcDd. 

In  this  case  the  Fi  generation  is  100  per  cent  heterozy- 
gous, all  four  factor  sets  being  heterozygous,  and  there- 
fore it  is  very  vigorous.  In  later  generations,  as  is  well 
known,  more  or  less  homozygous  sets  will  be  split  off. 
Introducing  homozygous  sets  into  some  individuals  will 
reduce  the  aggregate  heterozygous  condition  of  the  whole 


Hybrid  Vigor  163 

population  to  something  less  than   too  per  cent;   there 
will  therefore  be  a  corresponding  loss  in  \ig(jr. 

If  the  genotype  of  the  F^  population  be  considered  (a  simpler 
example,  AABBXaabb,  will  suffice),  some  very  clear  conclusions 
may  be  drawn.  The  F2  population  is  heterogeneous  with  respect 
to  hybrid  vigor,  in  sharp  contrast  with  the  F,,  where  all  the  individ- 
uals showed  the  same  amount  of  hybrid  vigor.  In  the  F^  there 
will  be  one  genotype  which  is  heterozygous  with  respect  to  all  of 
the  factor  pairs  involved  (as  was  the  F,),  and  which,  therefore, 
shows  the  maximum  amount  of  vigor.  There  will  be  other  geno- 
types which  are  homozygous  with  respect  to  all  the  factor  pairs, 
and  show  no  vigor.  And  there  will  be  still  other  genotypes  which 
are  parti}''  heterozygous  and  partly  homozygous,  and  show  an 
intermediate  amount  of  vigor.  This  heterogeneity  of  the  F,  gen- 
eration with  respect  to  amount  of  hybrid  vigor  is  in  agreement 
with  the  actual  experimental  results. 

If  the  average  vigor  of  the  whole  F^  population  be  computed, 
in  terms  of  relative  numbers  of  factor  sets  in  the  heterozygous 
condition,  this  will  be  found  to  have  a  value  of  50  per  cent,  in  con- 
trast with  the  o  per  cent  of  the  original  grandparental  generation 
and  the  100  per  cent  of  the  Fj.  On  the  same  basis  the  F3  will  be 
found  to  have  25  per  cent,  the  F4  12.5  per  cent,  and  so  on,  exactly 
one-half  of  the  vigor  being  lost  with  each  succeeding  generation  of 
inbreeding.  This  serves  to  account  for  Shull's  observation  that 
the  greatest  loss  in  vigor  is  between  the  Fi  and  F,  generations. 
Thereafter  the  loss  gradually  approaches  the  limit  when  the  per- 
fectly homozygous  condition  is  reached  for  the  whole  population, 
and  then  there  can  be  no  more  loss  in  vigor. 

3.  A  cross  between  sister  and  brother  elTects  nothing. 
— This  is  evident,  for  it  introduces  no  heterozygosity. 

4.  "A  cross  between  ])lants  belonging  to  two  self- 
fertilized  families  results  in  a  i)ri)geny  of  as  great  \igor, 
size,  and  productiveness  as  are  possessed  by  families 
that  have  never  been  self-fertilized."  Heterozygosis 
accounts  for  this  by  showing  that  a  cross  between  two 


164  Outline  of  Genetics 

pure  lines  may  bring  into  the  hybrid  a  maximum  number 
of  heterozygous  sets,  quite  as  many  as  are  present  in 
cross-fertilized  families. 

5.  Reciprocal  crosses  are  equivalent.^ — This  would 
obviously  follow  from  any  Mendelian  hypothesis  such 
as  heterozygosis. 

East  next  studied  tobacco  as  representing  those 
species  which  are  generally  self-fertilized  in  nature.  It  is 
a  common  impression  that  tobacco  is  a  striking  exception 
in  the  matter  of  hybrid  vigor.  In  tobacco  crosses  the 
hybrid  progeny,  instead  of  being  more  vigorous,  are  fre- 
quently less  vigorous  than  either  parent.  East  admits 
that  there  are  certain  cases  of  this  kind,  but  points  out  a 
number  of  other  cases  which  are  quite  "  normal"  in  show- 
ing hybrid  vigor.  In  any  event,  the  tobacco  situation 
strongly  suggests  the  idea  that  hybrid  vigor  appears  less 
prominently  in  species  that  are  generally  self-fertilized 
in  nature  than  in  species  normally  cross-fertilized. 

It  may  be  that  the  "subnormal"  tobacco  hybrids  are  products 
of  such  wide  crosses  that  hybrid  vigor  can  no  longer  operate  (see 
p.  169). 

The  phenomenon  of  hybrid  vigor  appears  also  in  a 
great  many  other  plants.  It  has  of  course  been  noted 
most  frequently  in  cultivated  forms,  but  there  is  also 
some  evidence  as  to  its  occurrence  among  wild  plants. 
Not  only  has  it  been  observed  among  many  angiosperms, 
woody  as  well  as  herbaceous,  but  also  among  gymno- 
sperms  and  pteridophytes;  and  there  is  even  some  slight 
evidence  that  hybrid  vigor  occurs  in  the  sporophyte  of 
the  bryophytes  (see  Britton  i). 

As  for  the  exact  nature  of  the  phenomenon,  quite 
a  number  of  features  arc  involved.     Primarily,  hybrid 


Hybrid  Vliior  165 

\iuj()r  amounts  to  an  increase  in  the  si/r  of  tells,  as  well 
as  multiplication  in  the  number  of  cells;    in  oilier  words, 
an  increase  in  the  ]X)wer  of  assimilation.     \'ial)ilit\'  of 
seeds  is  increased,  and  the  more  rapid  growth  and  earlier 
maturity  of  the    seedlings    is    quite    noticeable.     'I'ime 
of  flowering  and  maturing  is  hastened,  although  in  many 
cases    increased    longevity    has    been    brought    about. 
One   sees   a   distinct  increase   in  the   size  of  the  roots. 
In  the  stem  there  is  no  increase  in  the  number  of  nodes, 
but  the  internodal  development  is  striking.     (The  gain 
in   size  in   plants  which  are  more  or   less  determinate 
in    their   number  of  parts  is  made   up  of  an  increase 
in  the  size  of  parts  rather  than  in  the  number  of  parts.) 
Usually  the  stem  growth  is  greater  than  the  leaf  growth, 
but  the  increase  of  the  latter  can  be  definitely  traced. 
The  size  of  the  flower  is  usually  not  affected,  nor  is  there 
any  change  in  the  size  of  small  fruits,  such  as  tobacco. 
In  fleshy  fruits,  however,  such  as  tomato  and  egg  plant, 
there  is  a  marked  increase.     On  the  individual  plant 
there  are  distinctly  more  flowers  and  fruits,  and  in  some 
cases  separate  inflorescences  are  longer,  as  in  the  ears  of 
corn.     (Total  yield  in  corn  has,  in  some  crosses,  been 
increased   100  per  cent  or  more.)     Endurance  against 
unfavorable    environmental    factors    and    resistance    to 
disease  have  also  been  frequently  noted  as  properties  of 
hybrids.     Facility  of  vegetative  pro])agation  is  increased. 
(Moreover,  there  is  no  evidence  to  i)ro\e  that  plants  lose 
any  of  their  hybrid  \  igor  in  long  continued  vegetative 
multiplication  through  innumerable  generations.) 

In  general,  there  is  similarity  between  hybrid  \  igor 
and  the  effect  of  a  good  environment.  Those  characters 
which  arc  the  quickest  to  be  mcKliiied  by  external  factors 


1 66  Outline  of  Genetics 

also  show  the  greatest  change  on  crossing.  There  is  at 
least  one  difference  between  the  two,  however;  in  time 
of  maturity,  environment  and  hybrid  vigor  have  some- 
what opposite  effects.  Generally  speaking,  favorable 
growing  conditions  tend  to  delay  flowering  and  maturing, 
while  conditions  w^hich  tend  to  stunt  the  plants  tend, 
like  hybrid  vigor,  to  hasten  them  (East  and  Jones  6). 

There  seems  little  doubt  that  hybrid  vigor  is  also  manifested 
in  the  animal  kingdom.  One  might  reasonably  expect  this  from 
the  fact  that  the  principles  of  inheritance  are  fundamentally  the 
same  in  plant  and  animal  kingdoms,  and  hybrid  vigor  is  a  matter 
of  inheritance.  As  a  matter  of  fact,  there  are  many  cases  among 
the  records  of  professional  animal  breeders  which  might  be  cited 
as  evidence  of  hybrid  vigor.  It  seems  equally  evident,  however, 
that  this  is  not  so  general  a  phenomenon  among  animals  as  among 
plants;  and  it  should  be  noted  that  many  zoologists  refuse  to 
recognize  in  hybrid  vigor  anything  like  a  general  law,  pointing 
out  cases  among  animals  in  which  hybridizing  apparently  results 
in  loss  of  vigor. 

It  is  rather  to  be  expected  that  such  a  general  phenomenon 
as  hybrid  vigor  must  have  played  a  part  in  the  evolution  of  the  plant 
kingdom.    A  few  suggestions  follow  (from  East  and  Jones  6) . 

1.  Fixation  of  characters  favoring  cross-fertilization. — "Vari- 
ations must  have  appeared  that  favored  cross-fertilization. 
Those  plants  producing  a  cross-fertilized  progeny  would  have  had 
more  vigor  than  their  self-fertilized  relatives.  The  crossing 
mechanism  could  then  have  become  homozygous  and  fixed,  while 
the  advantage  due  to  cross-fertilization  continued." 

2.  Fixation  of  sex  act  itself. — "Some  means  of  favoring  union 
of  dissimilar  spores  occurred  as  a  chance  variation.  Through  the 
combination  of  somewhat  different  qualities  this  new  dual  product, 
the  zygote,  was  better  enabled  to  develop  and  reproduce.  Its 
survival  coefficient  was  high.  The  tendency  for  union  of  spores 
persisted  and  became  characteristic  of  the  species." 

3.  Preservation  of  undesirable  characters  in  cross-fertilized 
species. — "In  self-fertilized  species,  new  characters  that  weakened 


Hybrid  Vit^or  167 

the  individual  woidd  have  been  immediately  eliminated.  Oidy 
strains  that  stood  by  themselves,  that  survived  on  their  own  merits, 
would  have  been  retained.  On  the  other  hand.  weak,  j^enotyfxjs  in 
cross-fertilized  species  were  retained  through  the  vigor  that  they 
exhibited  when  crossed  with  other  genotypes.  The  result  is, 
therefore,  that  self-fertilized  strains  that  have  survived  competi- 
tion are  inherently  stronger  than  cross-fertilized  strains.  On  this 
account  weak  genotypes  may  often  be  isolated  from  a  cross- 
fertilized  species  that  as  a  whole  is  strong  and  hardy." 

4.  Rise  of  the  sporophyte  generation. — The  commonly  ac- 
cepted interpretation  of  hybrid  vigor  is  based  upon  a  Mendelian 
mechanism  that  would  be  effective  only  in  the  diploid  generation. 
In  the  evolution  of  the  plant  kingdom,  the  haploid  gametophyte 
generation  has  been  superseded  in  dominance  by  the  diploid  sporo- 
phyte generation.     Hybrid  vigor  may  help  to  account  for  this. 

Some  recent  investigations  have  extended  the  scope 
of  hybrid  vigor  in  an  interesting  and  significant  way. 
The  work  was  done  originally  by  Collins  and  Kemptox 
(3),  and  later  confirmed  and  extended  by  Jones  (8).  In 
brief,  it  is  as  follows. 

If  corn  sporophytes  exhibit  hybrid  vigor,  will  the 
endosperm  also  show  the  same  phenomenon  ?  Endo- 
sperms, as  has  been  stated,  are  genetically  equivalent 
to  sporophytes  in  several  ways.  If  crossing  increases 
vigor  and  size  of  sporophytes,  therefore,  it  might  be 
expected  to  increase  the  size  of  the  endosperms  also. 

Furthermore,  the  endosperms  have  considerable  advantage 
over  sporophytes  as  material  for  such  investigation.  We  say  that 
hybrid  sporophytes  are  more  vigorous  than  pure  bred  sporophytes, 
but  just  how  much  more  vigorous  cannot  be  stated  with  exactness. 
In  order  to  demonstrate  this  clearly,  it  would  be  necessary  to  have 
the  hybrid  and  the  pure  bred  stock  growing  side  by  side  in  e.xactly 
the  same  conditions,  but  the  conditions  cannot  be  controlled 
with  exactness.  The  environmental  factors  affecting  the  size  and 
vigor  of  a  corn  plant  are  numerous,  complex,  and  to  a  large  extent 


1 68  Outline  of  Genetics 

uncontrollable.  Thus  two  different  plants,  growing  side  by  side, 
might  be  in  a  distinctly  different  environment  without  the  fact 
being  recognized.  It  cannot,  therefore,  be  said  with  much  cer- 
tainty just  how  much  hybrid  vigor  a  given  plant  shows  when  there 
are  so  many  unknown  factors  that  might  affect  size  and  vigor. 
On  the  other  hand,  if  it  is  claimed  that  the  endosperm  of  one  grain 
shows  a  given  amount  of  hybrid  vigor  as  compared  with  the  grain 
that  grows  next  to  it  upon  the  same  ear,  the  statement  would  be 
more  exact,  for  the  two  endosperms  have  developed  under  con- 
ditions which  are  unquestionably  much  more  constant  than  the 
conditions  surrounding  the  different  sporophytes  in  a  corn  field. 

Jones  selected  a  plant  with  white  endosperm  and 
pollinated  it  with  a  mixture  of  its  own  pollen  and  pollen 
from  a  yellow  endosperm  race.  In  the  resulting  ear, 
therefore,  he  had  a  mixture  of  yellow  endosperm  grains 
and  white  endosperm  grains.  The  former  grains  of 
course  wxre  hybrid,  since  the  yellow  factor  was  introduced 
by  the  foreign  pollen,  while  the  white  endosperm  grains 
must  have  resulted  from  own  pollen  and  were  homozygous. 
In  this  way,  Jones  obtained  side  by  side  in  the  same  ear 
endosperms  obviously  hybrid  and  endosperms  obviously 
homozygous.  When  he  weighed  these  two  types  he 
found  that  the  hybrids  exceeded  the  homozygotes  in 
weight  by  from  5  to  35  per  cent. 

He  made  the  reciprocal  cross,  using  the  same  mixture 
of  yellow^  and  white  pollen  on  silks  of  the  yellow  race. 
Of  course  all  the  resulting  endosperms  were  yellow,  but 
the  hybrids,  which  had  the  yellow  factor  only  from  the 
female  side,  were  distinctly  lighter  yellow  than  the 
homozygotes,  which  had  the  yellow  factor  from  both 
male  and  female  sides.  Weighing  these  two  types, 
Jones  obtained  the  same  results  as  before,  the  hybrids 
exceeding  the  others  in  weight  by  an  average  of  20  per 


Hybrid  Vigor  169 

cent.  This  is  really  the  clearest  demonslralion  of  iiybrid 
vigor  that  has  ever  been  pro\'ide(l,  for  the  conditions  of 
the  experiment  were  ideally  constant. 

It  is  interesting  to  note  in  this  connection  that  there  is  no 
selective  action  favoring  foreign  pollen  when  these  pollen  mix- 
tures are  applied.  In  fact,  the  results  indicate  that  own  pollen 
is  successful  in  bringing  about  fertilization  in  a  slightly  greater 
number  of  cases  than  is  foreign  pollen. 

It  has  been  stated  that  the  amount  of  hybrid  vigor  varies 
directly  with  the  width  of  the  cross.  Of  course  this  statement 
applies  only  within  certain  limits.  The  situation  is  somewhat 
clarified  by  considering  the  following  series  of  cases  which  is 
arranged  with  respect  to  width  of  cross. 

1 .  Parents  so  diverse  that  cross  cannot  be  made. 

2.  Cross  possible  but  seed  obtained  fails  to  germinate.  E.\- 
ample,  certain  Nicotiana  crosses. 

3.  Hybrid  seed  germinates,  but  resulting  hybrid  plants  are  so 
weak  that  they  fail  to  reach  maturity.  E.xample,  other  Nicotiana 
crosses. 

4.  Hybrid  plants  mature  and  are  extremely  vigorous,  but  are 
sterile  except  possibly  in  back  crosses.  Example,  cabbage-radish 
hybrid,  an  enormous  but  completely  sterile  plant.  Example  from 
animal  kingdom,  the  mule.  (On  this  matter  see  also  chapter  on 
"Sterility.") 

5.  Hybrid  plants  more  vigorous  than  parents,  and  completely 
fertile.     Example,  corn  crosses  and  many  others. 

6.  Parents  to  cross  so  closely  related  that  no  protluction  of 
hybrid  vigor  is  noticeable. 

(An  interesting  phenomenon  appears  in  certain  wheat  crosses, 
where  it  is  found  that  the  Fi  endosperms  are  well  developed  in  the 
fertile  crosses,  but  shriveled  in  those  crosses  which  are  to  produce 
sterile  or  partially  sterile  F,  plants.  Even  in  these  latter  cases, 
however,  hybrid  vigor  appears  in  the  vegetative  parts  of  the  F, 
plants.     Sax  ii.) 

Obviously,  it  is  only  within  the  limits  of  classes  4  and  5  that 
it  can  be  said  that  hybrid  vigor  varies  directly  with  the  width  of 
the  cross.     It  is  impossible  to  say  where  the  species  boundary  tits 


170  Outline  of  Genetics 

into  the  foregoing  scheme,  since  species  boundaries  are  more  often 
matters  of  personal  opinion  than  indices  of  crossabihty. 

The  theory  of  heterozygosis  claims  that  hybrid  vigor 
appears  in  proportion  to  the  number  of  factors  in  which 
the  parents  of  the  cross  differ.  This  claim  should  be 
considered  briefly.  Is  heterozygosis  really  an  explana- 
tion of  the  phenomenon  of  hybrid  vigor?  It  seems 
obvious  that  it  is  not.  It  was  known  that  hybrids  were 
vigorous  because  they  were  hybrids.  Heterozygosis 
states  that  hybrids  are  vigorous  to  the  degree  that  their 
parents  differed  in  hereditary  factors;  in  other  words, 
this  is  merely  a  statement  that  hybrids  are  vigorous 
because  they  are  hybrids,  with  the  addition  that  the 
more  hybrid  a  hybrid  is  the  more  vigorous  it  is.  It 
follows,  therefore,  that  heterozygosis  is  not  an  explana- 
tion of  hybrid  vigor,  but  merely  a  restatement  of  the 
phenomenon  in  Mendehan  terms,  with  the  additional 
idea  that  there  may  be  various  degrees  of  hybrid  vigor. 
It  is  not  the  intention  to  discredit  heterozygosis  as  a 
valuable  conception,  but  to  point  out  that  it  is  not  a  real 
explanation,  merely  a  more  intelligent  statement  of  facts. 

Furthermore,  heterozygosis  is  rather  unsatisfactory 
in  another  way.  It  locks  the  door  on  any  hope  of  origi- 
nating pure  strains  having  as  much  vigor  as  first  genera- 
tion hybrids. 

For  these  reasons  it  would  seem  desirable  to  seek  an 
explanation  of  hybrid  vigor  along  other  lines.  Such  an 
explanation  may  be  developed  from  the  following  con- 
siderations. 

In  nature  a  "struggle  for  existence"  occurs  among 
species  and  individuals.  There  must  occur  also  a  struggle 
for  existence  among  unit  characters.     If  a  unit  char- 


Ilyhy'id   l^ii^or  171 

acter  is  umlcsinil^lc  il  is  cliniiiiatt'd,  Un-  tin-  iii<li\i(lual 
or  species  that  carries  it  is  elimiiialed.  This  would 
obviously  apply  particularly  lo  the  <l(>niinanl  characlers, 
for  undesirable  recessives  might  well  sur\  i\  l'  hv  escapinj^ 
natural  selection  while  in  heterozygous  combination  with 
their  dominant  allelomorphs.  It  follows  that  the  domi- 
nant unit  characters  that  have  survived  and  a])pear  in 
the  plants  of  today  are  for  the  most  part  desirable  ones. 

The  question  may  be  raised  as  to  what  constitutes  a 
''desirable''  character.  It  may  be  any  one  of  a  number 
of  things,  but  is  there  not  some  feature  which  is  common 
to  all  desirable  characters  ?  The  common  feature  of  all 
desirable  characters  would  seem  to  arise  from  their  rela- 
tion to  the  vigor  of  the  organism.  Each  desirable  char- 
acter must  add  somewhat  to  the  vigor  of  the  plant  that 
contains  it,  and  associated  with  vigor  are  such  things  as 
size  and  productiveness.  Is  it  not  reasonable  that  those 
plants  will  be  most  vigorous  which  have  in  combination 
the  greatest  number  of  desirable  characters  ?  1'he 
plants  which  have  the  greatest  combination  of  such 
characters  are  the  hybrids. 

A  diagram  similar  to  that  which  was  used  to  explain 
heterozygosis  may  be  considered: 

Parents  Fi 

A  A  BBCCDD  X  A  A  BBCCdd  =  A  A  BBCCDd  =  1  it  Uc  hybric  1  vigor 
AABBCCDDXAAbhccdd     =  A  A  BbCcDd  =  si'iW  more  hybrid  vigor 

In  that  explanation  it  was  stated  that  the  lirst  case 
showed  little  hybrid  vigor  because  it  had  only  one  hetero- 
zygous set  {Dd),  while  the  other  case  showed  more 
hybrid  vigor  because  it  had  three  such  heterozygous  sets. 
Hybrid  vigor,  therefore,  appeared  in  proportion  to  the 


172  Outline  of  Genetics 

number  of  heterozygous  sets  in  the  hybrid.  This  dia- 
gram served  the  purpose  of  explaining  heterozygosis, 
but  it  win  now  be  discarded  because  it  does  not  represent 
the  most  important  result  when  two  races  are  crossed. 
The  important  result  is  represented  in  the  following 
diagram : 

Parents  F« 

mbbuDDEEFF  ]AaBbCcDdEeFf=...or^  hybrid  vigor 

tifcCDDei     }  ^^imoDdeefj   =  less  hybrid  vigor 

The  thought  is  that  in  each  of  these  two  cases  the  hybrid 
is  more  vigorous  than  either  parent,  not  because  it  con- 
tains more  heterozygous  sets,  but  because  it  contains 
more  dominant  factors,  which  means  more  ''desirable" 
characters.  For  example,  in  the  first  case  each  parent 
contains  three  factors,  the  small  letters  representing 
merely  the  absence  of  factors.  The  Fi  generation,  there- 
fore, contains  six  factors,  and  for  this  reason  is  more 
vigorous  than  either  parent.  It  is  stated  in  the  diagram 
that  in  the  first  case  there  is  ''more  hybrid  vigor"  and 
in  the  second  case  "less  hybrid  vigor,"  simply  because 
hybrid  vigor  is  a  relative  term.  It  represents  merely 
how  much  more  vigorous  the  hybrid  is  than  either  parent. 
In  the  first  case  the  parents  have  three  factors  and  the 
hybrid  six,  the  increase  being  three,  which  measures  the 
amount  of  hybrid  vigor.  In  the  second  case  each  parent 
has  two  and  the  hybrid  four;  the  increase,  therefore, 
is  only  two,  and  for  this  reason  there  is  less  hybrid  vigor 
in  the  second  case  than  in  the  first. 

Assuming  that  the  majority  of  dominant  factors  are 
desirable,  and  that  desirable  factors  make  for  general 


Hybrid  Vigor  173 

vigor,  it  would  follow  tlial  the  most  vigorous  plant  will 
be  the  one  containing  the  greatest  number  of  dominant 
factors.  It  has  been  shown  that  the  j)lants  contain- 
ing the  greatest  number  of  dominant  factors  are  the 
hybrids;  it  is  for  this  reason  that  hybrids  are  relati\ely 
vigorous. 

The  following  question  may  be  raised.  If  it  is  granted  that 
most  desirable  factors  tend  somewhat  to  increase  the  general  vigor, 
do  they  all  do  this  to  the  same  degree  ?  The  natural  answer  is  in 
the  negative,  but  this  has  no  bearing  upon  the  validity  of  the 
explanation.  On  the  other  hand,  if  heterozygosis  be  accepted  for 
an  explanation  the  question  presents  a  dilhculty.  Heterozygosis 
would  suggest  that  Aa  induces  vigor,  not  because  of  any  particular 
factor  that  it  represents,  but  because  it  is  a  heterozygous  set. 
It  seems  more  reasonable  and  natural  to  suppose  that  certain 
factors  induce  more  vigor  than  others. 

It  is  evident  that  the  suggestion  made  above  is  that 
of  a  real  explanation  of  hybrid  vigor  and  not  merely  a 
restatement.  Keeble  and  Pellew  (id)  suggested  it  in 
1 9 10,  and  since  that  time  it  has  been  somewhat  discussed 
in  the  literature,  being  referred  to  as  ''the  hypothesis 
of  dominance  (accounting  for  hybrid  vigor)."  At  lirst 
statement  the  theory  seems  sound,  but  actually  it  does 
not  fit  the  facts.  The  two  outstanding  objections  to 
this  theory  of  dominance  are  brought  out  in  the  })ublica- 
tions  of  Shull,  Emerson,  and  East. 

I.  If  hybrid  vigor  were  due  to  dominance,  it  would  be 
possible  in  generations  subsequent  to  the  F,  lo  recom- 
bine  in  one  race  all  of  the  dominant  factors  in  the  homo- 
zygous condition.  Thus  there  could  be  isolated  a  race 
that  was  ''100  ])er  cent  vigorous,"  and  since  il  would 
be  homozygous,  its  vigor  would  not  be  lost  by  inbreeding. 
Actually,  though,  no  one  has  (as  yet)  been  able  to  "lL\" 


174  Outline  of  Genetics 

hybrid  vigor  in  this  way;  ''all  maize  varieties  lose  vigor 
when  inbred." 

2.  Experience  assures  us  that  the  distribution  of 
individuals  in  the  F2  generation  with  reference  to  hybrid 
vigor  is  represented  graphically  by  a  symmetrical  curve, 
similar  to  the  normal  probabilities  curve;  the  class  con- 
taining the  greatest  number  of  individuals  is  that  which 
shows  the  medium  amount  of  hybrid  vigor,  while  on 
either  side  of  this  class  the  fall  in  the  curve  is  regular, 
reaching  its  lowest  points  in  the  two  small  extreme  classes 
which  show  respectively  greatest  hybrid  vigor  and  least 
hybrid  vigor.  According  to  the  dominance  hypothesis, 
the  largest  class  of  the  F,  individuals  is  that  showing 
greatest  hybrid  vigor  (if  only  a  few  effective  factors  are 
assumed,  as  was  the  case  in  the  work  of  Keeble  and 
Pellew),  while  the  smallest  class  would  be  that  showing 
least  hybrid  vigor.  The  curve  representing  such  a  situa- 
tion would  be  asymmetrical  and  strikingly  different  from 
that  which  actually  occurs. 

For  these  two  reasons  the  dominance  hypothesis,  as 
proposed  by  Keeble  and  Pellew,  has  been  discarded. 
Although  it  is  theoretically  attractive,  its  failure  to  satisfy 
these  two  important  details  of  the  hybrid  vigor  situation 
has  condemned  it. 

Recently  Jones  (7)  has  ingeniously  modified  the 
dominance  hypothesis  so  as  to  avoid  these  difficulties. 
The  argument  is  essentially  the  same,  with  one  very 
significant  modification.  Jones  visualizes  the  situation 
as  represented  in  fig.  27.  In  this  case  it  is  a  question 
of  linkage  of  dominants  and  recessives.  The  vigor  of 
one  parent  is  due  to  the  two  dominant  factors  A  and  D, 
while  that  of  the  other  parent  is  due  to  the  two  dominant 


Hybrid  Vigor 


175 


factors  C  and  B.  The  hybrid  is  more  \i^'()r{)us  than 
either  because  it  combines  all  four  dominant  factors. 
The  attractiveness  of  this  scheme  Hes  in  the  fact  that  it 
escapes  the  objections  that  were  made  to  the  older  domi- 
nance hypothesis. 


Fig.   27. — Diagram  to  aid  in  visualising  Jones's  explanation  of 
hybrid  vigor  by  dominance  of  linked  factors. 


I.  The  fact  that  100  per  cent  hybrid  vigor  cannot  be 
fixed  is  quite  in  accordance  with  Jones's  scheme,  for  it 
is  obviously  impossible  to  isolate  a  race  homozygous  for 
all  four  factors,  A,  B,C,  and  D. 

As  a  matter  of  fact,  it  would  be  theoretically  possible  under 
this  scheme  to  isolate  just  such  a  homozygous  race.  If  crossing 
over  took  place  during  gamete  formation  by  the  F,.  .1  and  C  might 
come  to  lie  on  the  same  chromosome.  When  a  gamete  containing 
such  a  chromosome  mated  with  another  gamete  of  the  same  sort, 
a  race  would  thereby  be  established  which  was  homozygous  with 
respect  to  -1  and  C.  If  a  similar  performance  took  place  (cither 
simultaneously  or  in  some  subsequent  generation)  in  the  other 
chromosome  pair,  the  race  would  also  achieve  homozygosity  with 
respect  to  B  and  D,  and  would  thereafter  breed  true  for  all  four 
factors. 

It  must  be  evident,  however,  that  there  would  be  but  a  remote 
chance  of  realizing  this  theoretical  possibility.  in;ismuch  as  there 
actually  must  be  many  more  than  two  chromosome  pairs  involved, 


176  Outline  of  Genetics 

and  more  than  two  effective  factors  on  each  chromosome.  At  the 
same  time  it  is  rather  encouraging  to  believe  that  such  a  possibility 
exists,  so  that  eventually  we  may  be  able  to  obtain  a  race  that  is 
homozygous  for  all  or  practically  all  of  the  desirable  factors. 

2.  A  simple  mathematical  demonstration  will  show 
that  the  distribution  of  F,  individuals  is  quite  what  it 
should  be,  represented  by  a  symmetrical  curve  similar  to 
the  curve  of  probabilities. 

It  can  be  demonstrated  rather  rapidly  that  Jones's  scheme 
will  satisfy  the  requirements  on  distribution  of  F2  individuals 
with  respect  to  hybrid  vigor,  and  on  the  progressive  loss  of  vigor 
in  the  later  inbred  generations.  It  was  seen  that  the  heterozy- 
gosis theory  could  account  for  these  facts  by  the  use  of  the  simple 
example,  AAbbXaaBB.  As  a  matter  of  fact,  fig.  27  becomes  the 
exact  mathematical  equivalent  of  this  example  if  we  substitute 
the  ^c  chromosome  of  the  dominance  scheme  for  A  of  the  heterozy- 
gosis scheme;  the  bD  chromosome  for  b;  the  aC  chromosome  for 
a;  and  the  Bd  chromosome  for  b.  In  this  way  it  will  be  discovered 
that  the  two  schemes  run  exactly  parallel  in  accounting  for  the 
facts  in  every  generation. 

In  fact,  this  new  theory,  ''the  dominance  of  linked 
factors,"  seems  altogether  sound  and  natural.  We 
should  rather  expect  that  each  chromosome  would  bear 
several  dominant  factors  conducive  to  vigor  and  several 
recessives  as  well. 

Recently  Collins  (2)  has  presented  some  interesting  consid- 
erations bearing  on  this  scheme  of  Jones's.  Collins  maintains 
that  in  explaining  hybrid  vigor  one  should  place  the  emphasis  on 
the  suppression  of  deleterious  recessive  characters  rather  than  on 
the  accumulation  of  dominant  growth  factors.  This  is  merely  a 
change  in  the  point  of  view.  Collins  further  maintains,  however, 
that  the  dominance  scheme  can  really  account  for  the  facts  with- 
out the  assumption  of  linkage,  provided  a  sufiicient  number  of 
effective  factors  be  assumed.  There  is  some  rather  good  evidence 
to  support  these  contentions  of  Collins.     For  the  present,  how- 


Hybrid  Vigor  177 

ever,  Jones's  explanation  seems  distinctly  more  serviceable  than 
any  other  that  has  been  offered. 

Skull's  scheme  to  take  practical  advantage  of  hybrid  vigor 
in  such  a  plant  as  corn  has  one  distinct  drawback.  As  was  brought 
out  in  chapter  ii,  the  size  of  a  corn  plant  is  limited  by  the  size  of 
the  seed  which  produces  it,  and  this,  in  turn,  is  limited  by  the  size 
of  the  mother-plant  upon  which  the  seed  developed.  Since 
Shull's  races  A  and  B  were  both  inbred  races,  they  must  have  been 
rather  small.  Consequently,  whichever  wa>'  the  cross  was  made, 
the  seeds  containing  the  Fi  embr>'os  would  be  limited  in  size  by  the 
small  size  of  the  female  parent,  so  that  the  P',  plants  would  get  a 
poor  start  and  would  never  be  able  to  attain  the  size  that  would 
have  been  possible  had  they  come  from  large  seeds. 

Jones  (9)  suggests  a  way  of  overcoming  this  dilTiculty. 
Starting  with  four  pure  races,  cross  .1  with  B  and  C  with  I).  This 
will  result  in  two  vigorous  Fi  types,  either  one  capable  of  develop- 
ing large  seeds.  Cross  these  two,  and  u  double  hybrid  results 
which  combines,  in  good  part,  the  advantages  of  all  four  of  the 
original  races,  and  is  not  limited  in  size  by  starting  from  a  small 
seed.  Of  course  there  is  a  certain  drawback  here  also,  since  the 
generation  used  for  the  crop  is  an  F2  with  respect  to  the  two  original 
crosses,  and  has  somewhat  less  than  the  maximum  vigoi  on  that 
account.  The  relative  advantages  and  disadvantages  of  such 
breeding  plans  can  be  evaluated  only  by  experiment.  Junes 
claims  to  have  gotten  better  results  from  his  double  cross  method 
than  could  be  obtained  from  Shull's  plan. 

From  the  discussion  that  has  been  preseiUeci  in  lliis 
chapter,  one  may  safely  conclude  that  the  ])henomena 
which  arise  in  connection  with  inhreedin*^  and  outbreed- 
ing can  be  explained  satisfactorily  in  terms  of  the  Men- 
dehan  mechanism  of  inheritance.  It  should  l)e  recog- 
nized that  inbreeding  is  not  injurious  per  se  (^throuL^di  mere 
fact  of  consangtiinity),  but  because  it  serves  to  isohite 
undesirable  recessive  tyi)es  from  a  liybrid  nu'xture. 

The  proof  on  this  point  is  that  inl)rec'diiig  in  homozy- 
gous stock  results  in  no  deterioration,     joxios  (9)  has 


178  Outline  of  Genetics 

carried  on  through  twelve  generations  the  inbred  corn 
cultures  that  were  started  by  East.  In  the  course  of 
this  experiment,  a  great  number  of  undesirable  recessive 
types  have  been  thrown  off.  On  the  other  hand,  certain 
of  the  lines  that  have  been  isolated  by  this  inbreeding 
are  quite  normal  and  healthy,  though  small  in  stature 
and  yield.  A  point  of  homozygosity  has  been  reached 
where  further  inbreeding  brings  no  further  loss  in  vigor. 

On  the  other  hand,  hybrid  \'igor  does  not  arise  from 
the  act  of  crossing  per  se,  but  merely  through  a  combina- 
tion in  the  hybrid  of  the  maximum  number  of  desirable 
factors. 

On  this  point  the  proof  lies  in  the  fact  that  crossing 
brings  hybrid  vigor  only  when  the  parents  to  the  cross 
differ  in  their  germinal  constitution.  There  is  plenty  of 
evidence  on  this  point.  In  Jones's  inbreeding  experi- 
ments, a  point  of  homozygosity  has  been  reached  where 
crosses  between  different  individuals  of  the  same  line 
brings  absolutely  no  hybrid  vigor. 

In  conclusion,  attention  should  be  called  to  the  danger 
of  confusing  phenomena  of  hybrid  vigor  with  those  of 
cumulative  factors.  Both  mechanisms  may  operate  on 
some  generalized  quantitative  character  such  as  size,  but 
the  hereditary  behavior  is  distinctly  different.  Cumula- 
tive factors  bring  an  Fi  which  is  no  more  variable  than 
either  parent-type  and  intermediate  in  size,  and  later 
generations  which  are  highly  variable.  The  average  size 
of  the  whole  population,  however,  is  the  same  for  every 
generation,  including  the  parental  and  the  Fi  generations. 
The  hybrid  vigor  mechanism  also  brings  an  Fi  no  more 
variable  than  either  parent-type  (as  would  any  Mendelian 
mechanism  for  that  matter),  and  later  generations  which 


Hybrid  Vigor  179 

are  widely  variable.  In  this  case,  however,  the  average 
size  of  the  whole  population  is  distinctly  different  in  the 
different  generations.  In  such  a  matter  as  size  the 
hybrid  vigor  manifestations  would  he  superimposed  upon 
the  cumulative  factor  manifestations. 

It  might  appear  unsatisfactory  and  arbitrary  to  assume  domi- 
nance of  factors  as  essential  to  explaining  hybrid  vigor,  and  lack 
of  dominance  in  the  case  of  cumulative  factors.  It  is  quite  likely, 
however,  that  the  fundamental  and  ''natural"  distinction  between 
the  two  mechanisms  lies  in  this  very  point.  Where  a  number  of 
factors  interact  in  affecting  some  quantitative  character  and  those 
factors  show  lack  of  dominance,  a  cumulative  factor  mechanism 
is  thereby  set  up.  A  similar  interaction  where  the  factors  are 
dominant  brings  into  play  the  hybrid  vigor  mechanism.  There 
is  a  difference  between  the  two  mechanisms  simply  because  some 
factors  show  dominance  and  others  do  not. 

This  idea  may  be  reinforced  by  the  following  theoretical  sug- 
gestion. Where  the  environment  (using  the  term  in  its  widest 
sense)  imposes  no  limitation  upon  the  degree  to  which  a  character 
may  be  expressed,  it  follows  that  two  doses  of  a  factor  must  have 
twice  the  effect  of  one;  dominance  is  lacking.  Where  the  environ- 
ment limits  the  expression  of  a  character,  and  one  dose  of  a  factor 
results  in  a  development  of  the  character  to  this  limit,  two  doses 
can  affect  nothing  more;  dominance  is  present.  Furthermore, 
these  environmental  limitations  may  shift  as  the  environment 
changes.  Such  an  environmental  shift  could  affect  in  no  way  the 
degree  of  development  of  those  characters  in  connection  with 
which  there  is  no  dominance,  but  would  be  expected  to  affect  the 
degree  of  development  of  those  characters  where  dominance 
occurred.  According  to  this  idea,  we  should  expect  non-domi- 
nance or  cumulative  factor  characters  to  be  of  such  a  sort  that  the 
environment  never  affects  the  degree  of  their  development;  while 
dominance  or  hybrid  vigor  characters  would  be  those  which  envi- 
ronmental changes  could  also  modify.  lH>r  the  most  part  this 
actually  agrees  with  the  facts  (see  p.  165).  Further  investigation 
will  doubtless  provide  a  more  definite  answer  on  this  matter. 


i8o  Outline  of  Genetics 

LITERATURE  CITED 

1.  Brixton,  E.  G.,  A  hybrid  moss.     Plant  World  1:138.  1898. 

2.  Collins,  G.  N.,  Dominance  and  the  vigor  of  first  generation 
hybrids.     Amer.  Nat.  55:116-133.  jig.  i.  1921. 

3.  Collins,  G.  N.,  and  Kempton,  J.  H.,  Effects  of  cross-pollina- 
tion on  the  size  of  seed  in  maize.  U.S.  Dept.  Agric.  Circular 
124.  1913. 

4.  Darwin,  C,  The  effects  of  cross-  and  self-fertilization  in  the 
vegetable  kingdom.     London,  1876. 

5.  East,  E.  M.,  and  Hayes,  H.  K.,  Heterozygosis  in  evolution 
and  in  plant  breeding.  U.S.  Dept.  Agric,  Bur.  PI.  Ind.  Bull. 
243.  pp.  68.  pis.  8.  1912. 

6.  East,  E.  M.,  and  Jones,  D.  F.,  Inbreeding  and  outbreeding. 
Philadelphia.  1919. 

7.  Jones,  D.  F.,  Dominance  of  linked  factors  as  a  means  of 
accounting  for  heterosis.     Genetics  2:466-479.  191 7. 

8. ,  Bearing  of  heterosis  upon  double  fertilization.     Bot. 

Gaz.  65:324-333.  figs.  3.  1918. 
9.  ■ — ,  The  effects  of  inbreeding  and  cross-breeding  upon 

development.     Conn.  Agric.  Exp.  Sta.  Bull,  207.  1918. 

10.  Keeble,  F.,  and  Pellew,  C,  The  mode  of  inheritance  of 
stature  and  of  time  of  flowering  in  peas  {Pisum  sativum). 
Jour.  Genetics  i :  47-56.  19 10. 

11.  Sax,  Karl,  Sterility  in  wheat  hybrids.  I.  Sterility  relation- 
ships and  endosperm  development.  Genetics  6:399-416. 
1921. 

12.  Shull,  G.  H.,  Hybridization  methods  in  corn  breeding. 
Amer.  Breeders  Mag.  1:98-107.  1910. 


CHAPTER  XIII 
SEX  DETERMINATION 

Sex  determination  is  very  properly  a  part  of  the  sub- 
ject of  genetics.  It  should  be  realized,  however,  that  a 
vast  amount  of  investigation  has  been  carried  on  in  this 
field,  and  it  will  be  possible  here  to  take  up  only  a  limited 
number  of  representative  cases. 

Since  this  subject  has  been  investigated  a  great  deal 
more  thoroughly  and  for  a  great  many  more  years  in 
animals  than  in  plants,  it  will  be  appropriate  first  to 
consider  some  of  the  findings  of  the  zoologists.  Until 
very  recently,  at  least,  there  appeared  two  outstanding 
and  seemingly  quite  contradictory  views  as  to  the  basis 
of  sex  determination. 

1.  Some  believed  that  sex  is  predetermined  by  the 
chromosome  equipment  that  enters  into  the  zygote. 

2.  Others  believed  that  sex  may  be  determined  othcr- 
w^ise  than  by  the  chromosomes,  the  decisive  factors  being 
certain  physiological  conditions  surrounding  the  unfer- 
tilized egg  or  the  developing  embryo. 

These  two  general  views  will  ])e  referred  to  as  the  chro- 
mosome theories  and  the  physiological  theories.  Repre- 
sentative examples  of  each  will  be  considered  brietly. 

Chromosome  theories. — A  classic  example  of  the 
simplest  kind  is  to  be  found  in  the  nematode  worms. 
Fig.  28  will  indicate  how  sex  is  determined  in  this  case. 
Both  male  and  female  have  ten  chromosomes  (com- 
monly   called   aulosomcs)    to   determine    most    of    their 

181 


l82 


Outline  of  Genetics 


somatic  characters;  but  in  addition  there  are  extra 
chromosomes  that  determine  sex,  known  as  sex  chromo- 
somes, or  heterochromosomes .  In  this  case  the  male  con- 
tains only  one  sex  chromosome,  while  the  female  con- 
tains two.     At  the  reduction  division,  when  the  gametes 


MALE 

OOOO 

o  •  o 
oooo 

MATURE  INDIVIDUALS 


i 


FEMALE 

DOOO 
©••  O 

OOOO 

, — - — ■ > 

OOOO 

o  •  o 
oo  oo 


oo  oo 
o»  ^o 
oooo 


Zygotes 


(This  type  of  zygote  (This  type  of  zygote 

produces  a  male)  produces  a  female) 

Fig.  28. — Illustrating  behavior  of  sex  chromosomes 


are  formed,  the  chromosome  equipment  is  reduced  one- 
half.  It  is  obvious  that  in  the  female  each  egg  receives 
one  sex  chromosome,  and  therefore  all  eggs  are  alike  in 
this  feature.  In  the  male,  however,  with  one  sex  chro- 
mosome, at  the  reduction  division  the  solitary  sex  chro- 
mosome goes  to  one  pole,  leaving  the  other  pole  without 


Sex  Dclcrui'nuUiou  183 

such  a  chromosome.  As  a  result  lluTc  arc  two  kinds  of 
sperms,  one  haU'  conlaininj^^  a  sex  chromosome,  the  other 
half  containing  none.  At  fertilization,  if  an  egg  mates 
with  a  sperm  having  a  sex  chromosome  the  zygote  con- 
tains two,  and  this  will  develop  into  a  female,  for  females 
are  characterized  by  two  sex  chromosomes.  With  a 
sperm  of  the  other  type,  the  zygote  receives  only  one  sex 
chromosome  and  must  produce  a  male  individual.  As  a 
result,  males  and  females  are  produced  in  equal  numbers, 
sex  being  determined  by  the  type  of  sperm  that  enters 
into  the  sex  fusion. 

Certain  conclusions  may  be  drawn  from  this  mecha- 
nism of  sex  determination,  which  will  serve  to  provide 
a  sharp  contrast  with  the  corresponding  conclusions  that 
may  be  drawn  from  the  physiological  theories. 

a)  The  sex  ratio  will  regularly  be  50  per  cent  males: 
50  per  cent  females.  It  would  be  rather  hopeless  to 
modify  this  ratio  by  artificial  means. 

h)  Sex  is  a  qualitative  matter,  only  two  conditions 

being  possible,  strictly  male  and  strictly  female. 

Numerous  instances  of  the  sex  chromosome  mechanism  have 
been  discovered  in  the  animal  kingdom.  Details  ditler  in  the 
different  cases,  but  the  essential  mechanism  remains  the  same. 
In  addition  to  the  type  of  case  described  above,  where  the  male- 
has  only  one  member  of  the  sex  chromosome  pair,  there  are  in 
general  three  other  possibilities.  The  male  may  have  one  largo 
chromosome  (similar  to  the  pair  in  the  female)  paired  with  a  small 
one;  the  male  may  have  two  sex  chromosomes  of  ai^proximately 
the  same  size  but  different  in  shape;  or  the  male  may  have  two 
sex  chromosomes  which  are  morphologically  itientical,  but  physi- 
ologically different  in  their  intluence  on  sex.  In  all  of  these  cases 
the  fundamental  mechanism  remains  the  same,  the  male  being 
heterozygous  for  sex,  so  that  two  types  of  six^rms  are  protluced  in 
equal  numbers,  and  the  sex  of  the  olYspring  depends  upon  which 


184  Outline  of  Genetics 

type  of  sperm  has  effected  fertilization.  In  the  cases  where  the 
male  has  an  unequal  pair  of  sex  chromosomes,  that  member  of  the 
pair  which  is  similar  to  the  equal  pair  of  the  female  is  known  as 
the  X  chromosome,  while  the  other  chromosome  of  the  male  is 
the  Y  chromosome  {XX  is  female,  XY  is  male). 

Furthermore,  although  the  male  is  usually  the  heterozygote 
for  sex,  there  are  some  cases  in  which  the  female  is  the  heterozy- 
gote. In  such  cases  the  sperms  are  all  alike;  two  types  of  eggs 
are  produced  in  equal  numbers,  and  the  sex  of  the  offspring  depends 
upon  which  type  of  egg  has  effected  fertilization.  This  is  the 
situation  in  the  birds  and  in  the  Lepidoptera  group  of  insects,  while 
in  practically  all  of  the  other  known  cases  it  is  the  male  that  is  the 
heterozygote  for  sex. 

When  genes  are  located  on  the  X  chromosome  their  method  of 
inheritance  is  characteristic,  being  known  as  sex-linked  inheritance. 
(This  term  should  not  be  confused  with  sex-limited  inheritance, 
which  applies  to  cases  where  the  genes  are  carried  on  the  auto- 
somes in  the  usual  manner,  but  can  express  themselves  only  in  one 
sex,  e.g.,  heavy  beardedness  in  man.)  Numerous  examples  of 
sex-linked  inheritance  are  to  be  found  in  the  fruit  fly. 

The  female  fruit  fly  has  a  pair  of  X  chromosomes,  while  the 
male  has  an  X  mated  with  a  F,  the  two  being  distinguishable  by 
their  shape.  Numerous  genes  are  located  on  the  X  chromosome, 
but  none  have  been  identified  on  the  F,  which  seems  quite  negli- 
gible in  inheritance.^  "Vermilion"  {v),  a  recessive  to  normal  red 
eye  (F),  is  a  gene  of  this  sort.  When  a  vermilion  female  {iX) 
{vX)  is  mated  with  a  red  male  {VX)  F,  all  of  the  female  progeny 
are  bound  to  be  red  {vX)  {VX),  w^hile  all  of  the  male  progeny  will  be 
vermilion  {vX)  F,  as  is  explained  by  fig.  29.  This  has  often  been 
called  "zig-zag"  inheritance,  since  the  character  of  the  mother 
passes  to  the  sons,  while  the  character  of  the  father  passes  to  the 
daughters.  All  of  the  other  possible  matings  work  out  just  as  one 
would  expect  from  the  mechanism  that  is  involved. 

» Some  very  recent  papers  (Castle  lo)  have  suggested  that  genes 
may  actually  be  located  on  the  F  chromosomes  in  some  animals.  A 
peculiar  "one-sided"  type  of  inheritance  results,  since  it  is  possible  for 
such  genes  to  be  present  only  in  the  male.  The  author  is  not  as  yet 
certain  that  these  findings  will  be  "accepted"  by  other  geneticists. 


Sex  Dclcrminaliou 


i8  = 


In  this  connection  it  will  be  worth  while  to  consider  brielly 
some  of  the  work  of  Bridges  (5),  which  provides  the  "final" 
demonstration  that  the  chromosomes  are  the  bearers  of  hereditary 
characters,  and  that  the  sex  chromosomes  are  really  the  effective 


Fig.  29. — Shaded  X  chromosomes  carries  gene  (V)  for  red  eye; 
unshaded  X  chromosome  lacks  this  gene  (i.e.,  condition  v)]  V  chromo- 
some of  male  distinguished  by  shape.  Individuals  carrying  l'  are 
red  eyed;  those  lacking  it  are  vermilion  eyed.  This  diagram  shows  how 
vermilion  female  (upper  left)  mated  with  red  male  (upper  right)  results 
in  red  daughters  (lower  left)  and  vermilion  sons  (lower  right). 


units  in  determining  sex.  Occasional  non-disjunction  of  the  A' 
chromosome  during  gametogcnesis  in  a  vermilion  female  results  in 
the  production  of  two  abnormal  types  of  eggs.  (t'A')  (rA')  and  O 
(fig.  30).     The  matings  of  these  two  abnormal  types  of  eggs  with 


1 86 


Outline  of  Genetics 


the  two  normal  types  of  sperms  from  a  red  male  will  result  in  four 
possible  types  of  zygotes: 

A  {vX)  {vX)  egg  with  a  {VX)  sperm  gives  a  {vX)  iyX)  {VX) 
zygote,  which  might  be  expected  to  produce  a  red  female,  but 
actually  dies  (according  to  Bridges'  earlier  statements). 

A  {vX)  {vX)  egg  with  a  Y  sperm  gives  a  {vX)  {vX)  Y  zygote, 
which  produces  a  vermilion  female. 


6 

Fig.  30. — Showing  the  four  types  of  zygotes  which  result  from  mat- 
ings  between  non-disjunctional  eggs  of  vermilion  female  and  normal 
sperms  of  red  male.  Note  particularly  the  second  and  third  z\-gotes, 
which  produce  the  "exceptional"  individuals,  vermilion  female  and  red 
male. 

An  O  egg  with  a  (VX)  sperm  gives  a  (VX)  zygote,  which 
produces  a  red  male. 

An  O  egg  with  a  Y  sperm  gives  a  Y  zygote,  which  dies. 

Inasmuch  as  vermilion  female  crossed  with  red  male  normally 
gives  only  red  daughters  and  only  vermilion  sons,  the  vermilion 
daughters  and  red  sons  which  result  from  the  non-disjunctional 
eggs  appear  as  startling  exceptions  to  the  normal  rule.     The 


Sex  Dclcrmi)ialion  187 

occurrence  of  occasional  individuals  of  these  exceptional  tyix's  led 
Bridges  to  suspect  that  non-tlisjunction  had  taken  j)lace.  Later 
he  effected  a  striking  confirmation  by  cytological  demonstration 
that  the  exceptional  vermilion  females  possessed  a  Y  chromosome, 
and  that  the  exceptional  red  males  had  no  Y  chromosome.  This 
work  provides  the  linal  convincing  demonstration  that  the  chromo- 
somes are  the  bearers  of  hereditary  characters,  since  abnormalities 
in  the  distribution  of  a  certain  chromosome  set  arc  accomjjanied 
by  corresponding  abnormalities  in  the  distribution  of  those  genes 
which  were  assumed  to  be  located  on  that  chromosome  set. 

In  the  chapter  on  bud  variation  the  phenomenon  of  ''chromo- 
some elimination"  was  discussed  (p.  121).  Morgan*  and  Bridges 
(17)  have  discovered  this  sort  of  thing  in  connection  with  the  sex 
chromosomes  in  the  fruit  fly.  An  individual  which  starts  its 
development  as  a  normal  female,  A'A',  has  one  of  the  A'  chromo- 
somes eliminated  from  one  of  the  daughter-cells  at  an  early 
embryonic  division.  Tissues  arising  from  this  daughter-cell 
have  only  one  A"  chromosome  and  show  the  characteristics  of  the 
male  sex,  while  the  rest  of  the  tissues  are  female.  Individuals  of 
this  part  female — part  male  type  are  known  as  gynamiromorphs. 

Physiological  theories.— In  1906  Hertweg  (14) 
performed  some  sex  determination  experiments  with 
frogs.  The  eggs  are  laid  free  in  the  water  before  fer- 
tilization, so  that  they  furnish  unusually  good  material 
for  such  experiments.  Normally  the  eggs  are  fertilized 
very  soon  after  they  are  laid,  with  the  result  that  the 
progeny  consists  of  api^roximately  50  j)er  cent  males  and 
50  per  cent  females.  Hertweg  took  some  of  these  eggs 
and  allowed  them  to  overripen  before  fertilization  took 
place,  that  is,  he  put  aside  some  eggs  as  soon  as  they 
were  laid  and  allowed  them  to  remain  unfertilized  iov  an 
unusually  long  period.  While  these  eggs  were  standing 
in  the  water  he  found  that  they  absorbed  an  unusual 
amount  of  water,  and  the  obvious  conclusion  was  that 
overripe  eggs  show  high  water  content.     He  then  allowed 


1 88  Outline  of  Genetics 

these  overripe  eggs  to  be  fertilized,  and  the  resulting 
progeny  were  loo  per  cent  males.  His  conclusion  was 
that  sex  was  not  determined  by  the  chromosome  equip- 
ment, but  by  the  physiological  conditions  of  the  egg, 
high  water  content  resulting  in  males. 

This  theory  was  confirmed  in  a  striking  way  in  191 2 
by  Miss  King  (15),  who  performed  the  converse  of 
HERT^VEG's  experiments,  using  toads'  eggs.  Taking 
some  newly  laid  eggs,  she  withdrew^  water  from  them; 
then  allowed  them  to  be  fertilized,  and  the  resulting 
progeny  were  90  per  cent  females.  The  obvious  conclu- 
sion is  that  eggs  with  low  water  content  produce  females. 

Finally,  there  is  the  remarkable  work  of  Riddle  (18) 
with  pigeons.  Hertweg  and  Miss  King  had  found  that 
sex  is  determined  by  the  physiological  factor  of  water 
content.  Riddle  has  investigated  the  matter  a  little 
more  fully,  and  from  his  analysis  of  the  physiological 
conditions  of  male  and  female  he  gives  the  following 
contrasts : 

Male  Female 

High  percentage  of  water  Low  percentage  of  water 

Low  percentage  of  fat  High  percentage  of  fat 

Low  percentage  of  phosphorus  High  percentage  of  phosphorus 

High  rate  of  metabohsm  Low  rate  of  metaboUsm 

It  appears  from  this  that  high  water,  low  fat,  and  low 
phosphorus  are  male  attributes  or  conditions,  while  the 
female  attributes  are  the  reverse.  The  main  feature  of 
difference,  however,  to  which  the  other  contrasting  con- 
ditions are  subordinate,  is  that  the  male  shows  high 
metabolism  and  the  female  low  metabolism.  The  idea 
is  that  any  physiological  conditions  that  affect  water 
content,  fat  content,  or  phosphorus  content,  or  through 


Sex  Dctcrniiiititio)!-  189 

these  (or  otherwise)  the  metaboh'c  rate  in  the  egg,  will 
affect  the  sex  of  the  resulting  ])rogeny. 

Following  these  ideas,  Riddle  was  able  to  control 
the  sex  ratio  by  various  means.  Furthermore,  he  makes 
the  somewhat  startling  statement  that  sex  is  a  quanti- 
tative phenomenon;  that  is,  the  difference  between  male 
and  female  is  a  difference  in  degree  only.  A  diagram 
(fig.  31)  will  illustrate  the  situation.  It  rej)re.sents  a 
graduated  scale  based  on  the  physiological  condition  of 

High  H,0  U.  H,0 


Males  with                        Females  witK 

Low  Fat 

Very  Male                   Some  Female                       Some  Male                       Veiy  Female 
Males                       Characteristics                    Oiaractemtics                         Females 

H.(h  F„ 

LowP 

/                             \/                               \ 

H>(hP 

'v                                                                 ^'^                                   " 

High 

miI5                                                 '^"'ALES 

L<i- 

Metabolism 

MrtaUia 

Fig.  31. — Illustrating  Riddle's  idea  of  sex.  Sexes  differ  only  quanti- 
tatively, and  it  is  possible  to  find  various  degrees  of  maleness  and  female- 
ness  at  different  points  along  the  scale. 

the  egg.  The  egg  may  be  at  any  point  on  the  scale,  and 
the  sex  of  the  individual  produced  by  the  egg  will  depend 
upon  its  position  on  the  scale.  An  egg  in  any  position 
to  the  left  of  the  middle  results  in  a  male  and  to  the  right 
in  a  female.  It  should  be  noted  that  if  the  egg  is  near 
one  of  the  extremes  the  j^rogeny  will  be  either  a  vcn,' 
masculine  male  or  a  very  feminine  female;  while  if  the 
egg  lies  near  the  middle  point,  on  one  side  or  the  other, 
the  progeny  will  be  a  male  with  some  female  character- 
istics, or  a  female  with  some  male  characteristics;  in 
other  words,  a  feminine  male  or  a  masculine  female. 
In  fact  Riddle  was  actually  able  to  bring  this  about, 
obtaining  at  will  males  with  all  degrees  of  maleness,  etc. 


iQo  Outline  of  Genetics 

This  mechanism  of  sex  determination  suggests  the 
following  conclusions,  which  contrast  sharply  with  the 
corresponding  conclusions  that  were  drawn  from  the 
sex  chromosome  mechanism: 

a)  The  sex  ratio  has  no  fixed  value,  but  may  be 
modified  artificially  through  manipulation  of  the  effec- 
tive physiological  conditions. 

h)  Sex  is  a  quantitative  matter,  '^ strictly  male" 
and  ''strictly  female''  being  merely  the  tw^o  extremes, 
between  which  there  may  occur  various  grades  of  "inter- 
sexes" or  ''sex  intergrades." 

Having  as  a  background  these  two  contrasting  the- 
ories on  sex  determination  in  animals,  w^e  may  consider 
briefly  some  of  the  situations  that  have  been  uncovered 
in  the  plant  kingdom.  A  few  meager  bits  of  evidence 
suggest  a  sex  chromosome  mechanism  in  plants. 

Strasburger  (22)  has  described  some  experiments 
with  the  liverwort  SphaerocarpHS,  which  is  peculiarly 
favorable  material  for  such  work.  It  is  "dioecious," 
like  many  liverworts,  but  a  remarkable  feature  is  that 
the  spores  hang  together  in  the  tetrad.  Ordinarily 
when  spores  mature  the  tetrads  are  no  longer  distinguish- 
able. Sowing  such  free  spores,  one  may  get  the  50-50 
ratio  of  male  and  female  gametophytes,  but  this  is  no 
sure  indication  that  the  sexes  are  evenly  divided  in  every 
tetrad;  it  may  have  been  only  an  equal  division  in  the 
capsule  as  a  whole.  Sphaerocarpus,  howxver,  provided 
an  opportunity  to  test  this  matter,  for  one  could  isolate 
mature  individual  tetrads,  the  four  spores  hanging 
together.  When  such  tetrads  were  sown  in  separate 
pots,  four  gametophytes  were  obtained  in  most  cases, 
and  practically  always  two  of  the  gametophytes  were 


Sex  Dclcrminalioi  191 

male  and  the  other  two  female.  This  beliaNior  certainly 
suggests  an  even  separation  of  the  sexes  at  the  reduction 
division,  such  as  would  be  hrouf^ht  about  by  the  sex 
chromosome    mechanism. 

More  recently  Allen  (i),  after  repeating'  and  con- 
firming the  foregoing  experiment,  made  a  systematic 
cytological  search  for  the  X  chromosome  in  Sphacro- 
carpus.  He  now  reports  that  one  large  chromosome 
(X),  exceeding  in  length  and  thickness  the  other  chro- 
mosomes, characterizes  the  cells  of  the  female  game- 
tophyte,  while  the  cells  of  the  male  gametophyte  are 
characterized  by  one  very  small  chromosome  (!').  His 
investigation  shows  that  in  spore  formation  two  of  the 
spores  of  the  tetrad  receive  the  large  chromosome,  while 
the  other  two  receive  the  small  chromosome. 

This  rather  clearly  establishes  a  sex  chromosome 
mechanism,  but  the  situation  is  distinctly  dilTerent  from 
that  in  animals.  The  sex  chromosome  mechanism  in 
animals  provides  for  a  differentiation  of  sexual  individ- 
uals in  the  diploid  generation,  the  female  being  A'A' 
and  the  male  XY .  The  sexual  individuals  in  Spluicro- 
carpus,  however,  are  of  the  haploid  gametophyte  gen- 
eration, the  female  gametophyte  regularly  being  A',  the 
male  F,  and  the  sexless  diploid  sporophyte  generation 
being  regularly  A'F.  In  the  animal  mechanism,  sex  is 
really  estabhshed  only  at  the  time  of  fertiHzation,  while 
in  Sphaerocarpus  it  is  established  immetliately  at  the 
reduction  division. 

Marchal  (16)  has  done  an  interesting  bit  of  work  which 
further  provides  indirect  evidence  on  a  sex  chromosome  mechanism 
for  sex  determination  in  the  gametophyte  generation.  I'utiaria 
is  a  ''dioecious"  moss,  and  hence  it  may  be  assumed  that  the  sexes 


192  Outline  of  Genetics 

are  separated  at  the  reduction  division  in  the  formation  of  spores. 
Each  spore  carries  the  potentiaHties  for  one  sex  only;  but  of  course 
the  sporophyte  as  a  whole  before  the  reduction  division  must  carry 
the  potentialities  for  both  sexes.  Marchal,  by  a  peculiar  tech- 
nique of  his  own,  clipped  a  fragment  from  a  young  sporophyte  and 
induced  it  to  reproduce  aposporously;  that  is,  the  sporophyte 
fragment  produced  a  gametophyte  directly.  The  fragment  must 
have  contained  the  potentiaHties  for  both  sexes,  since  it  consisted 
of  tissue  in  which  the  reduction  division  had  not  yet  occurred. 
Presumably,  the  resulting  gametophyte  should  be  bisexual,  pro- 
ducing both  antheridia  and  archegonia,  and  this  was  the  result 
actually  obtained.  It  is  quite  in  accord  with  the  sex  chromosome 
theory  and  a  striking  confirmation  of  it. 

If  one  is  to  find  in  plants  a  sex  chromosome  mecha- 
nism comparable  to  that  of  animals,  he  must  look  to  the 
cases  where  the  diploid  sporophyte  generation  shows  a 
sexual  differentiation  of  individuals,  such  as  in  dioecious 
angiosperms.  Santos  (19),  working  with  Elodea,  has 
show^n  that  in  the  tissues  of  the  male  plant  there  regularly 
occurs  (in  addition  to  certain  even  pairs  of  autosomes) 
one  uneven  pair  of  chromosomes,  of  which  the  larger 
member  may  be  designated  as  X  and  the  smaller  member 
as  Y.  He  has  further  demonstrated  that  the  reduction 
division  serves  to  separate  the  members  of  this  pair,  so 
that  half  of  the  pollen  grains  contain  an  X  chromosome 
and  the  other  half  contain  a  Y  chromosome.  PreHmi- 
nar^^  examination  suggests  that  the  tissues  of  the  female 
plant  are  regularly  of  the  XX  constitution,  but  this  part 
of  the  work  has  not  yet  been  completed.  There  seems 
Uttle  doubt,  however,  that  here  is  a  sex  chromosome 
mechanism  exactly  equivalent  to  those  found  in  the 
animal  kingdom.  Two  types  of  pollen  grains,  X  and 
Y,  produced  in  equal  numbers,  and  mating  with  (pre- 


Sex  Dctcrminatioi  193 

sumably)  one  type  of  egg,  X,  result  in  50  i)er  cent  female 
individuals,  A^.Y,  and  50  per  cent  male  individuals,  AT. 

Indirect  evidence  of  a  sex  chromosome  mechanism  in  angio- 
sperms  is  provided  by  some  of  the  experiments  of  Corrk.vs  and 
Shull.  Correns  (12)  crossed  the  dioecious  Bryonia  dioica  with 
the  hermaphroditic  B.  alba,  while  Shull  (21)  crossed  Lychnis 
dioica  with  hermaphroditic  mutants  from  the  same.  The  sexual 
behavior  of  the  progenies  in  the  two  cases  was  not  identical,  but 
both  suggested  a  sex  chromosome  mechanism  with  the  male 
heterozygous  for  sex.  (The  theoretical  explanations,  however, 
are  so  complex  and  dubious  that  they  cannot  conveniently  be 
discussed  here.) 

^lore  recently  Correns  (ii),  working  wiili  Melatulriiim 
{Lychnis),  has  uncovered  an  interesting  phenomenon  which 
might  be  interpreted  as  indicating  a  sex  chromosome  mechanism 
and  a  type  of  sex-linked  inheritance.  It  is  assumed  that  pollen 
grains  of  the  two  types  are  produced  in  equal  numbers,  but  that  the 
"female-determining"  grains  (X)  contain  a  gene  which  hastens 
pollen  tube  growth,  while  the  "male-determining"  grains  (Y)  lack 
this  gene.  When  a  deficient  amount  of  pollen  is  applied  to  the 
stigmas,  the  resulting  sex  ratio  is  44  per  cent  males:  56  per  cent 
females.  When  a  large  excess  of  pollen  is  applied,  so  that  com- 
petition between  "male-determining"  and  "female-determining" 
pollen  tubes  is  more  severe,  the  resulting  sex  ratio  is  32  per  cent 
males: 68  per  cent  females.  A  moderate  excess  of  pollen  results 
in  40  per  cent  males: 60  per  cent  females. 

At  the  present  date  there  is  probablx'  more  evidence 
to  support  physiological  theories  of  sex  detemiination 
in  plants.  It  should  be  borne  in  mind  that  the  majc^rit}' 
of  plants  are  bisexual  individuals,  and  that  such  cases 
are  hardly  comparable  with  unisexual  animals.  \'ery 
often  in  bisexual  plants  the  male  and  female  g;mietes 
are  produced  at  slightly  dilYerent  stages  in  the  life-cycle, 
and  the  interpretation  of  such  phenomena  is  usually 
sought  in  temis  of  plnsiological   conditions.     A  young 


194  Outline  of  Genetics 

fern  prothallium  frequently  produces  antheridia  only, 
while  a  mature  prothallium  produces  archegonia  only. 
In  attempting  explanation  it  is  usually  stated  that  more 
''nutrition"  is  required  for  the  production  of  archegonia 
and  eggs  than  for  the  production  of  antheridia  and 
sperms.  During  the  flowering  season,  monoecious  angio- 
sperms  (e.g.,  Begonia)  will  sometimes  produce  the  male 
flowers  distinctly  earlier  than  the  female  flowers,  or  the 
reverse.  In  connection  with  such  cases  botanists  usu- 
ally feel  that  the  potentialities  for  both  sexes  are  at  all 
times  present  in  all  the  tissues  of  the  individual,  and  that 
it  remains  for  some  unknown  complex  of  physiological 
conditions  to  call  out  one  or  the  other  sex  in  any  given 
region  of  the  plant.  Surely  no  sex  chromosome  mecha- 
nism can  be  at  play  to  account  for  sex  differentiation 
here!  Only  by  assuming  a  reduction  division  some 
time  during  somatogenesis  or  a  regular  and  peri- 
odic ''chromosome-elimination"  could  such  cases  be 
brought  in  line  with  the  sex  chromosome  mechanism 
of  sex  determination.  It  is  much  more  reasonable 
(for  the  present  at  least)  to  regard  bisexual  plants  as 
"outside  the  scope"  of  the  sex  chromosome  mechanism. 
In  unisexual  plants  one  is  confronted  by  a  different 
situation,  and  sometimes,  as  discussed  above,  a  sex  chro- 
mosome mechanism  seems  to  be  determining  sex.  Even 
here,  however,  it  would  doubtless  be  possible  to  cite  more 
evidence  favoring  the  physiological  theories.  Angio- 
sperms  that  are  normally  dioecious  have  frequently 
produced  bisexual  plants  that  might  well  be  regarded  as 
"intersexes."  Considerable  work  has  been  done  to  indi- 
cate that  various  environmental  conditions  may  either 
modify  the  sex  ratio  or  result  in  the  production  of  inter- 


Sex  Dclcrniinuliofi  195 

sexes.  A  favorite  subject  for  such  cxpcriincnts  is  Cuuua- 
bis,  and  many  investigators  have  succeeded  in  a  certain 
amount  of  artificial  manipulation  of  sex  in  this  fonn. 
ScHAFFNER  (20)  has  gone  so  far  as  completely  to  reverse 
the  sex  of  given  individuals  by  m()dif>'ing  the  cultural 
conditions.  He  voices  the  belief  of  many  other  botanists 
when  he  draws  the  following  conclusions: 

''Sexuality  is  a  state  or  condition  not  Mendelian  in 
nature,  but  related  to  functional  activity  of  the  plant 
and  profoundly  influenced  by  cn\'ironment.  Malcness 
and  femaleness  in  hemp  are  probably  controlled  by  the 
metabolic  level  of  the  cells,  and  sex  reversal  takes  place 
when  the  metabolic  level  is  decidedly  changed  or  dis- 
turbed. Any  tissue  in  its  growth  may  be  in  a  neutral 
state  of  varying  degrees  of  intensity,  and  during  its 
continued  growth  can  pass  from  one  state  to  the  other 
without  any  reference  to  chromosome  segregation  or 
combination  which  are  the  ordinary  causes  of  Mendel ian 
phenomena." 

The  situation  might  be  clarified  somewhat  b\  the 
following  generaUzation.  Not  only  are  there  relatively 
fewer  plants  than  animals  in  the  unisexual  condition, 
but  even  in  those  plants  that  arc  unisexual,  this  condition 
is  not  so  completely  ''established"  as  in  animals.  'J'iie 
sex  chromosome  mechanism  seems  to  operate  only  in 
organisms  where  the  purely  unisexual  condition  prevails 
and  has  prevailed  for  some  time  back  in  their  i)hylo- 
genetic  history.  Many  of  the  dioecious  angiosj)enns, 
however,  seem  rather  recently  to  have  been  deri\-e(l  from 
ancestors  which  have  the  two  se.xes  represented  in  the 
same  flower  (or  at  least  on  the  same  plant).  In  these  the 
dioecious  condition  seems  not  to  have  been  firmlv  estab- 


196  Outline  of  Genetics 

lished ;    a  regular  sex  chromosome  mechanism  has  not  as 
yet  been  perfected. 

It  is  small  wonder  that  bewildering  sex  conditions  appear 
in  these  "imperfectly  dioecious"  angiosperms.     Schaffner  and 
others  have  pointed  out  how  extrinsic  factors  may  operate  to 
determine  sex  in  such  forms.     It  is  evident,  though,  that  intrinsic 
hereditary  factors  may  also  play  their  part  in  such  cases.     This 
may  be  illustrated  by  some  of  the  experiments  of  Strasburger 
(22)  on  Mercurialis  (later  confirmed  by  Yampolsky  23).     Stras- 
burger had  the  idea  that  the  pollen  mother-cell  develops  pollen 
grains  with  stronger  and  weaker  male  tendencies,  while  the  mega- 
spore  mother-cell  develops  eggs  with  stronger  and  weaker  female 
tendencies.    It  is  therefore  the  algebraic  sum  of  the  two  as  they  meet 
in  fertilization  that  determines  the  sex  of  the  progeny.     If  a  pollen 
grain  with  strong  male  tendencies  mates  with  an  egg  with  weak 
female  tendencies  the  resulting  individual  will  be  male,  and  simi- 
larly for  the  other  combination.     These  assumptions  are  supported 
by  the  behavior  of  Mercurialis.    This  form  has  for  the  most  part 
pure  male  and  pure  female  individuals,  but  at  times  it  throws 
intersexes   of   various   grades.     Certain   plants   are   prevailingly 
female,  but  bear  a  few  "weak"  male  flowers.     In  a  plant  of  this 
sort,  it  would  seem  that  the  female  tendencies  are  stronger  than 
the  male.     When  such  a  plant  is  inbred,  using  pollen  from  the 
weak  male  flowers  on  the  stigmas  of  the  strong  female  flowers,  the 
resulting  progeny  is  all  female,  which  is  in  accordance  with  Stras- 
burger's  theory.     Other  plants  are  prevailingly  male,  but  bear 
a  few  weak  female  flowers,  and  inbreeding  these  results  in  all 
males.     Finally,  there  are  some  plants  which  are  evenly  monoe- 
cious, half  their  flowers  being  strong  males  and  the  other  half 
strong  females.     Inbreeding  such  plants  yields  a  progeny  which  is 
50  per  cent  male  and  50  per  cent  female.     It  is  obvious  that  from 
such  results  Strasburger  would  be  convinced  of  his  theory  of 
male  and  female  tendencies. 

Of  considerable  interest  to  botanists  is  some  work  that  has 
been  done  on  the  sexual  condition  of  Mucor  and  related  genera  of 
fungi.  Blakeslee  (2,  3)  found  three  different  sexual  types  of 
mycelia,  two  of  which  he  called  "plus"  and  "minus"  strains. 


Sex  Dclermi nation  igy 

Although  they  looked  alike  in  every  particular,  he  concluded  that 
they  were  sexually  different  for  the  following  reason.  Neither 
strain  by  itself  is  capable  of  producing  zygotes,  but  when  phis 
and  minus  strains  are  brought  together  sexual  branches  from  the 
one  meet  sexual  branches  from  the  other  and  pro<luce  abundant 
zygotes.  The  natural  conclusion  is  that  Blaki-:slee's  plus  and 
minus  strains  represent  the  male  and  female  conditions,  although 
the  sex  cannot  be  distinguished  by  direct  examination.  The  third 
type  of  mycelium  he  called  the  neutral  strain,  for  it  is  incapable 
of  producing  zygotes  in  any  combination. 

The  answer  to  the  question  as  to  where  sex  is  determined  in 
these  forms  is  as  follows.  WTien  a  zygote  germinates,  one  or  more 
sporangia  are  produced  very  early,  and  individuals  are  multi[)lied 
by  the  spores  from  these  sporangia.  In  Mucor  itself  the  segrega- 
tion of  sex  is  evidently  completed  before  the  formation  of  spores 
in  this  first  sporangium,  for  all  of  its  spores  will  produce  the  same 
strain  of  mycelium.  The  sporangium  as  a  whole,  therefore,  is 
either  male  or  female.  In  Phycomyces,  however,  a  dilTerent 
behavior  appears.  The  zygote  produces  a  sporangium,  but  the 
sporangium  is  not  completely  of  one  sex.  It  produces  three  types 
of  spores:  spores  producing  the  plus  strain,  spores  producing  the 
minus  strain,  and  spores  producing  the  neutral  strain.  The  plus 
strain  then  perpetuates  only  plus  strains  through  its  spores,  which 
means  that  sex  is  fixed  in  this  case.  The  minus  strain  behaves  in 
a  similar  manner.  The  neutral  strain,  however,  produces  spores 
of  all  three  types,  an  interesting  situation,  for  it  suggests  Mendelian 
segregation. 

BuRGEFF  (8)  has  performed  an  interesting  operation  on  this 
same  material.  By  means  of  a  very  careful  technique,  he  grafletl 
parts  of  the  plus  strain  on  to  the  minus  strain  and  secured  graft 
hybrids  with  the  characteristics  of  the  neutral  strain.  In  attempt- 
ing to  interpret  the  foregoing  results,  it  should  be  remembered  that 
Mucor  and  its  relatives  are  coenocytic.  so  that  nuclei  of  two  tyi>cs 
can  mingle  freely  in  the  mycelium. 

Proceeding  further  with  this  material.  Bi.akeslee  (4)  isolated 
numerous  plus  and  minus  strains,  and  found  that  they  ditTereti 
in  their  sexual  intensity,  as  computed  in  terms  of  the  number 
of  zygotes  formed  under  standard  conditions.     Evidently  some 


198  Outline  of  Genetics 

strains  are  more  strongly  plus  (female,  as  was  later  determined  by 
indirect  means)  and  others  less  strongly  plus,  and  the  same  was 
true  of  the  minus  (male)  strains.  This  strongly  suggests  a  quanti- 
tative interpretation  of  sex. 

Recently  Burgeff  (9)  has  discovered  some  startling  facts  in 
connection  with  sex  in  some  of  the  other  genera  of  Mucorineae. 
Absidia  shows  the  customary  plus  and  minus  strains,  as  does  also 
Parasitella.  This  latter  genus  is  a  parasite  upon  other  genera  of 
the  same  family,  and  in  connection  with  this  parasitic  habit  there 
appears  a  remarkable  situation.  The  plus  strain  of  Parasitella 
will  parasitize  the  minus  strain  of  Absidia  but  not  the  plus  strain 
of  Absidia;  while  the  minus  strain  of  Parasitella  will  parasitize 
the  plus  strain  of  Absidia  but  not  the  minus  strain  of  Absidia. 
The  author  concludes  that  the  hypothetical  sexual  substance  which 
distinguishes  the  plus  and  minus  mycelia  of  Absidia  is  identical 
with  the  substance  that  induces  parasitism,  and  that  the  parasitic 
relationship  here  has  arisen  as  the  result  of  an  unsuccessful  attempt 
at  hybridization  between  the  two  genera. 

The  discussion  to  date  leaves  the  interpretation  of 
sex  determination  in  a  distinctly  unsettled  condition. 
We  find  that  in  a  great  many  animals  and  a  very  few 
plants  a  very  definite  sex  chromosome  mechanism  oper- 
ates to  determine  sex;  and  that  sex  is  a  qualitative 
proposition,  only  the  two  conditions  of  strictly  male  and 
strictly  female  being  possible.  On  the  other  hand,  it  is 
suggested  by  the  sexual  behavior  of  some  animals  and 
quite  a  number  of  plants  that  the  general  physiological 
condition  is  important  in  determining  sex;  and  that  sex 
is  a  quantitative  matter,  intersexes  or  sex  intergrades 
being  possibilities  that  are  frequently  realized. 

There  are  three  possible  conclusions  with  reference 
to  these  contradictory  theories:  (i)  an  acceptance  of 
one  and  rejection  of  the  other;  (2)  the  claim  that  both 
amount  to  the  same  thing,  that  they  express  the  same 


Sex  Dctcrwiuation  iqq 

fundamental  facts  in  dilYcrcnt  tcnns  or  by  the  use  of 
different  indices;  (3)  the  chiim  that  both  are  true  hut 
cover  different  territories,  that  one  of  tJiem  cxphiins 
certain  types  of  cases  and  the  other  explains  other  t>'])cs 
of  cases. 

Until  very  recently  the  tJiird  alternative  seemed  the 
most  acceptable,  inasmuch  as  the  two  t\7)es  of  sex  deter- 
mining mechanism  had  never  been  clearly  identiticd  in 
the  same  organism.  The  recent  work  of  Hkiugk.s  (6,  7), 
however,  sways  opinion  to  the  second  of  the  foregoing 
alternatives,  for  it  harmonizes  the  two  contradictory 
views  on  sex  determination  to  a  degree  that  would  hardly 
have  seemed  possible. 

An  unexpected  distribution  in  inheritance  of  known 
factors,  which  are  located  on  the  second  and  third  chro- 
mosomes of  the  fruit  fly,  was  explainable  on  the  assump- 
tion that  the  female  parent  was  triploid  with  respect  to 
these  chromosomes.  Cytological  examination  proved 
that  this  was  actually  the  case.  The  same  grouj)  of 
flies  also  exhibited  some  remarkable  irregularities  in 
their  sex  condition.  A  considerable  group  of  intersexes 
occurred,  as  evidenced  by  the  secondary  sex  characters 
and  the  condition  of  the  gonads  as  well.  (This  was 
apparently  a  bimodal  group  ,  some  of  the  intersexes  being 
of  a  more  '^female"  t>pe  and  others  of  a  more  "male" 
t}T3e.)  Cytological  examination  of  these  indi\iduals 
revealed  that  the  second  and  third  chromosomes  were 
regularly  present  in  a  triploid  condition,  that  the  fourth 
chromosome  was  either  diploid  or  tri])loitI,  and  that  two 
X  chromosomes  were  regularly  present  (witJi  or  without 
a  Y  chromosome).  The  situation  is  interpreted  as 
follows : 


200  Outline  of  Genetics 

''It  is  not  the  simple  possession  of  two  X  chromosomes 
that  makes  a  female,  or  of  one  that  makes  a  male.  The 
preponderance  of  genes  that  are  in  the  autosomes  tends 
toward  the  production  of  male  characters;  and  the  net 
effect  of  genes  in  the  X  is  a  tendency  to  the  production 
of  female  characters.  The  ratio  of  2X:2  sets  autosomes 
produces  a  female,  while  iX :  2  sets  autosomes  produces  a 
male.  An  intermediate  ration,  2X13  sets  autosomes, 
produces  an  intermediate  condition,  the  intersex." 

''The  fourth  chromosome  seems  to  have  a  disproportionately 
large  share  of  the  total  male-producing  genes;  for  there  are  indica- 
tions that  the  triplo-fourth  intersexes  are  preponderantly  of  the 
'male'  type,  while  the  diplo-fourth  intersexes  are  mainly  'female' 
type." 

According  to  this  conception,  3 A':  2  sets  autosomes  should  be 
" superfemales "  and  iX:3  sets  autosomes  should  be  "supermales." 
Bridges  has  actually  identified  such  types,  both  being  sterile. 

It  is  certain  that  this  conception  will  exert  a  far-reaching  influ- 
ence upon  the  existing  ideas  of  sex  detemination.  In  the  first 
place,  it  gives  a  somewhat  more  exact  idea  as  to  the  elements 
effective  in  determining  sex.  Hitherto  it  had  been  thought, 
rather  vaguely,  that  the  X  chromosome  determines  sex  either  per 
se  or  by  virtue  of  some  special  factor  which  it  contains.  It  is 
interesting  now  to  realize  that  a  number  of  factors  may  be  influ- 
encing sex  in  one  direction  or  the  other,  and  perhaps  that  these  are 
identical  with  factors  which  have  previously  been  known  as  playing 
another  role.  A  different  rate  of  metabolism  has  commonly  been 
associated  with  the  two  sexes;  a  study  of  the  influence  of  specific 
factors  on  metabolic  rate  now  becomes  significant  in  this  connection. 

In  the  second  place,  it  furnishes  an  exact  interpretation  of 
intersexes  on  a  chromosome  basis.  Hitherto  intersexes  have 
usually  been  interpreted  in  rather  vague  physiological  terms,  and 
have  been  used  as  an  argument  against  the  sex  chromosome  theory 
(or  have  been  harmonized  with  the  sex  chromosome  theory  only 
by  the  assumption  of  some  additional  extra-chromosomal  influ- 
ence— GoLDSCHMiDT    13).     Bridges'    Conception    now    paints    a 


Sex  Dclcrniiniition  201 

quantitative  picture  of  sex  without  calling  upon  any  other  cffcclive 
elements  than  the  "orthodox"  factors  of  inheritance  that  arc 
located  on  the  chromosomes.  Intersexes  arc  therefore  accounted 
for  by  the  same  general  mechanism  as  normally  produces  only  pure 
males  and  females  in  the  fruit  fly. 

In  the  third  place,  the  theoretical  possibility  of  artificially 
controlling  sex  is  illuminated.  Such  control  should  l>e  possible  to 
the  degree  that  the  ordinary  heritable  characters  can  successfully 
be  duplicated  artificially.  Hridc;i:s  acknowledges  that  thr  en- 
vironment may  affect  sex  within  certain  limits.  .Mlhough  sex 
is  fundamentally  a  quantitative  proposition.  \  X  :  2  sets  autosomes 
provides  such  a  considerable  preponderance  of  male-inducing 
factors,  and  2X12  sets  autosomes  provides  such  a  preix)nderance 
of  female-inducing  factors,  that  only  these  two  distinct  qualitative 
conditions  are  visualized  under  ordinary  circumstances.  Hoth  of 
the  foregoing  conditions  are  far  from  the  point  of  ecjuilibrium 
between  the  opposite  types  of  sex  intluences.  Under  such  cir- 
cumstances the  minor  influences  of  single  factors  in  one  direction 
or  the  other  produce  no  appreciable  elTect.  .\s  a  matter  of  fact, 
a  factor  mutation  in  the  germ  plasm  or  an  unusual  combination 
of  extrinsic  physiological  conditions  might  intervene  to  influence 
a  male  individual  toward  femaleness  (or  vice  versa),  but  the 
individual  is  so  preponderantly  male  that  the  etTects  of  these 
minor  influences  are  not  noticeable. 

On  the  other  hand,  in  those  individuals  (the  interse.xes)  where 
the  male-inducing  and  female-inducing  factors  are  near  the  jxiint 
of  equilibrium,  the  minor  influences  of  single  factors  in  one  direc- 
tion or  the  other  become  noticeable.  In  such  an  intlividual  an 
unusual  combination  of  extrinsic  physiological  conditions  may 
swing  the  individual  more  toward  maleness  or  more  toward  female- 
ness, and  these  deviations  will  be  observeil.  This  idea  is  borne 
out  by  the  actual  facts,  since  the  inlluence  of  environmental 
conditions  upon  the  grade  of  sex  in  Bridgks'  intersexes  is  notice- 
able, but  the  same  conditions  do  not  produce  noticeable  clTecls 
upon  the  normal  males  and  females.  The  intersexes,  representing 
a  condition  near  an  equilibrium  between  opj)osite  factor  influ- 
ences, are  more  "responsive"  to  environmental  diflerenci's.  more 
"fluctuating"  than  are  the  normal  males  and  females. 


202  Outline  of  Genetics 

One  might  then  assume  that  in  organisms  where  the  unisexual 
condition  has  existed  for  some  time  back  in  phylogeny,  a  definite 
sex  chromosome  mechanism  has  been  established.  This  mecha- 
nism insures  (normally)  the  production  of  two  types  of  individuals 
in  equal  numbers,  those  which  are  preponderantly  males  and  those 
which  are  preponderantly  females.  The  grade  of  sex  does  not 
appear  to  fluctuate  in  response  to  varying  environmental  influences, 
since  these  influences  are  relatively  insignificant  in  such  cases. 
In  other  organisms,  however,  which  have  more  recently  been 
evolved  from  bisexual  ancestors,  a  regular  sex  chromosome  mecha- 
nism has  not  yet  been  perfected.  The  appropriate  machinery  is 
not  yet  at  work  to  produce  individuals  which  are  preponderantly 
male  and  individuals  which  are  preponderantly  female  in  equal 
numbers.  Instead,  sex  is  being  influenced  by  numerous  factors 
which  are  distributed  sporadically  rather  than  in  organized  groups 
as  in  the  fruit  fly.  The  net  effect  of  these  factor  influences  is 
commonly  near  to  the  point  of  equilibrium,  so  that  the  organism 
is  more  responsive  to  environmental  influences  on  sex  grade. 
Under  such  conditions  the  sex  grade  and  the  sex  ratio  may  be 
susceptible  to  a  certain  amount  of  artificial  control  through  manip- 
ulation of  the  effective  environmental  influences. 

Finally,  this  work  of  Bridges'  casts  a  new  light  upon  the  whole 
subject  of  unit  characters.  Careful  investigation  of  flies  which  are 
triploid  with  respect  to  one  or  more  chromosome  sets,  leads 
Bridges  to  draw  the  same  general  conclusions  with  regard  to 
other  so-called  unit  characters  that  he  drew  with  regard  to  the 
character  of  sex.  Many  characters  have  their  degree  of  develop- 
ment influenced,  not  merely  by  the  presence  or  absence  of  certain 
single  genes,  but  by  the  net  effect  of  the  influences  of  numerous 
genes.  It  is  true  that  there  is  commonly  one  gene  that  exerts  a 
greater  influence  on  the  character  in  question  than  do  any  other 
genes,  and  it  is  quite  common  that  all  the  other  genes  may  be 
constant  in  their  presence  or  absence,  so  that  only  the  effects  of 
the  one  gene  are  noticeable,  and  we  identify  it  as  " the  determiner" 
of  the  character  in  question.  In  such  cases  the  equiUbrium  of 
•opposing  influences  is  normaUy  being  affected  to  a  perceptible 
•degree  only  by  the  presence  or  absence  of  a  single  gene.  Abnormal 
situations,  however,  may  arise,  as  the  result  of  non-disjunction  of 


Sex  Dclermi)hitiou  203 

certain  chromosome  sets.  This.  I)y  inlro<iucing  a  relatively 
greater  number  of  genes  which  have  a  positive  inlluence  (or 
negative,  as  the  case  may  be),  may  modify  the  degree  of  expression 
of  the  character  in  question  so  that  it  shows  a  grade  not  previously 

seen. 

LITERATURE  Clli:i) 

1.  Allen,  Charles  E.,  A  chromosome  ditTercncc  correlated  with 
sex  differences.     Science  46:466-467.   1917. 

2.  Blakeslee,  A.  E.,  Sexual  reproduction  in  the  Mucorincae. 
Proc.  Amer.  Acad.  40:205-319.  1914. 

3.  ,   DifTerentiation  of  sex   in   thallus  gametophytc  and 

sporophytc.     Bot.  Gaz.  42: 161-178.   1906. 

4.  ,  Sex    in   Mucors.     Ann.    Rcpt.    (\arnegie  Inst.   1920: 

128-130. 

5.  Bridges,  C.  B.,  Non-disjunction  as  proof  of  the  chromosome 
theory  of  heredity.     Genetics  i :  1-52.   1916. 

6.  ,     Triploid     intersexes     in     Drosopliila     nielanogaster. 

Science  54:252-254.  1921. 

7.  ,  The  origin  of  variations  in  sexual  and  sex-limited 

characters.     Amer.  Nat.  56:51-63.  y/g^.  7.  192:. 

8.  BuRGEFF,  H.,  tJber  Sexuahtat,  \'ariabilitat.  und  N'crerbung 
bei  Phycomyces  nitens.  Ber.  Deutsch.  Bot.  (iesell.  30:679- 
685.  1912. 

9.  ,  Sexuahtat  und  Parasitismus  bei  ^lucorincen.     Ber. 

Deutsch.  Bot.  Gesell.  38:318-328.  192 1. 

10.  Castle,  W.  E.,  The  I'-chromosome  type  of  sex-linked  inherit- 
ance in  man.     Science  55:703-704.  1922. 

11.  CoRRENS,  C,  Die  Konkurrenz  der  mannlichen  und  dcr 
weiblichcn  KeimzcUen  und  das  Zahlenverhiiltnis  iler  beiden 
Geschlechter.     Naturwissenschaf ten  6:277-280.  1918. 

12.  ,  and  GoLDSCHMiDT,  R.,  Die  Nererbung  u.  Hestimmung 

des  Geschlechts.     Berlin.   1913- 

13.  GoLDSCHMiDT,  R..  Untcrsuchungcu  iiber  Interse.xualital. 
Zeit.  Indukt.  Abstamm.  \'ererb.  23:1-199.  ph.  ::.  figs.  S4. 
1920. 

14.  Hertweg,  R.,  \'erhandl.  Deutsch.  Ztx)l.  (iesell.  1906;  see 
also  Biol.  Centralbl.  32: 1.  191 2. 


204  Outline  of  Genetics 

15.  King,  H.  D.,  Jour.  Exp.  Zool.  12:19. 

16.  Marchal,  El.  et  Em.,  Aposporie  et  sexualite  chez  les  ^lousses. 
I,  II,  III.  Bull.  Acad.  Roy.  Belgique.  CI.  Sci.  1907.  765-789; 
1909.  1249-1288;  1911.  750-778. 

17.  Morgan,  T.  H.,  and  Bridges,  C.  B.,  Contributions  to  the 
genetics  of  Drosophila  melanogaster.  I.  The  origin  of  gynandro- 
morphs.  Carnegie  Inst.  Washington  Publ.  278.  ph.  4.  figs.  10. 
1919. 

18.  Riddle,  Oscar,  The  control  of  the  sex  ratio.  Jour.  Wash. 
Acad.  Sci.  7:319-356.  191 7. 

19.  Santos,  J.  K.,  UnpubHshed. 

20.  ScHAFFN^ER,  J.  H.,  Influence  of  environment  on  sexual  expres- 
sion in  hemp.     Bot.  Gaz.  71:197-219.  1921. 

21.  Shull,  G.  H.,  Reversible  sex  mutants  in  Lychnis  dioica.  Bot. 
Gaz.  52:329-368.  1911. 

22.  Strasbltiger,  E.,  Uber  geschlechtbestimmende  Ursachen. 
Jahrb.  Wiss.  Bot.  48:427-520.  1910. 

23.  Yampolsky,  Cecil,  Inheritance  of  sex  in  Mercurialis  annua. 
Amer.  Jour.  Bot.  7:21-38.  1920. 


INDEX 


INDEX 


Absidia,  sex  in,  198 

Acquired    characters,    inheritance 
of,  2,  12 

Aleurone  color  inheritance,  147 

Allard,  H.  A.,  132 

Allelomorphs,  39;    multiple,   112, 
119 

Allen,  C.  E.,  191 

Altenburg,  E.,  no 

American  evening  primrose,  muta- 
tion in,  6 

Anderson,  E.  G.,  125 

Anthocyanin,  59 

Antirrhinum,  crosses  in,  9;   muta- 
tions in,  no 

Ascaris,    differentiation    of    germ 
plasm  and  body  plasm  in,  14 

Autosomes,  i8i,  199 

Babcock,  E.  B.,  12 

Bateson.  W.,  16,  50,  59,  122,  126 

Baur,  E.,  109,  no,  122,  123 

Beans,  semi-sterility  in,  135 

Begonia,  sex  in,  194 

Belling,  John,  n4,  135 

Blakeslee,  A.  F.,  114,  11  s,  196,  197 

Blaringhem,  L.,  18 

Blending  inheritance,  53 

Bolley,  H.  L.,  21 

Bonnier,  Gaston,  29 

Bouvardia,  chimaeras  in,  122 

Boveri,  Th.,  14 

Bridges,  C.  B.,  no,  n^,  i  u,  1S5. 

187,  199 
Britton,  E.  G.,  164 
Br>'onia,  sex  determination  in,  193 
Bud  variations,  119 


Burbank,  L.,  150 
BurgelT.  II.,  197,  198 

Cabbage-radish     hybrid,     hybrid 
vigor  in,   169;    sterilily  in.   133 

Cannabis,  sex  determination  in,  195 

Capsella,  alpine  adaptation  in,  28 

Capsicum,  chlorophyll  inluTilancc 
in,  124 

Castle,  William  i:.,  2^,  88,  184 

Chimaeras,  1 2 1 

Chloroph\ll  inheritance.  1 2^ 

Chromogen,  59,  64,  67 

Chromosomes,  41 

Chromosome  aberrations,  113 

Chromosome  changes,  113 

Chromosome  elimination.  121,  187 

Chromosome  theories  of  sex  deter- 
mination, iSi 

Chrxsomdid  beetles,  dilTerentia- 
tion  of  germ  plasm  and  body 
plasm  in,  16 

Claussen,  R.  E.,  12  • 

Collins,  G.  X.,  1O7.  176 
Complementary  factors,  57,  67,  69, 

70,  137 
Complexnuitation.  U3.  no 
Compositae.    parthenogenesis    in 

Conscious  effort,  in  animal  evolu- 
tion, 2 
Constant  variations,  o 
Continuity  of  the  germ  plasm,  15 
Continuous  variations.  4 

Corn:  aleurone  layer  in.  147;  l)ud 
variation  in.  up;  chlorophyll 
inheritance  in,  125;  crossing 
over  in,  107;  factor  interactions 


207 


208 


Outline  of  Genetics 


in,  57,  6i,  64,  68,  69,  70,  78; 
false  inheritance  of  acquired 
characters  in,  31,  177;  hybrid 
vigor  in,  157,  161;  inheritance 
of  acquired  characters  in,  18; 
inheritance  of  endosperm  char- 
acters in,  145,  146,  148,  151; 
inheritance  of  quantitative e  char- 
acters in,  78;  linkage  in,  97, 
154;  multiple  allelomorphs  in, 
112,  119;  non-disjunction  in, 
154;  sterility  in,  132,  134 

Correlation,  physical,  81 

Correns,  C,  49,  53,  72,  125,  193 

Crossing  over,  102,  175 

Crypthybrids,  134 

Cumulative  factors,  67,  72,  177 

Cytoplasmic  segregation,  123 

Czapek,  P.,  and  M.  E.,  59 

Darwin,  Charles,  3,  11,  77,  87,  156 
Danvin,  Erasmus,  i 

Datura:    non-disjunction  in,  114; 

tetraploidy  in,  115;  triploidy  in, 

115 
Deficiency,  113,  116 
Degeneration  through  disuse,  3 
Determinate  variations,  9 
Determiner,  42,  56 
Detlefsen,  J.  A,,  107 

DeVries,  Hugo,  6,  11,  84,  86,  87, 
109,  no,  117 

Dictyota,  inheritance  of  acquired 
characters  in,  33 

Dihybrid  ratio,  46 

Diploid,  42 

Discontinuous  variations,  6 

Diseases,  inheritance  of,  20 

Dominance,    39;     accounting    for 

hybrid  vigor,  1 73 ;  failure  of,  53 

Doncaster,  L.,  14,  15 

Dose,  double  and  single,  44 

Double  crossing  over,  106 

Duplex,  50 

Duplication,  113,  116 


East,  E.  M.,  57,  61,  64,  78,  84,  93, 
130,  138,  148,  153,  157,  160,  166 

Elimination  factor,  22 

Elodea,  sex  determination  in,  192 

Emerson,  E,  R.,  61,  69,  78,  97, 

107,  119,  152,  153,  154 
Endosperm,   inheritance   in,    141; 

hybrid  vigor  in,  167,  169 

Engler,  Arnold,  32 

Envdronment,  inheritance  of  effects 
of,  23;  role  in  evolution,  2,  3 

Eyster,  W.  H.,  134 

Factor  hypothesis,  56,  93 

Farnham,  ]\I.  E.,  114 

Fern  prothallia,  sex  in,  194 

Fittest,  survival  of,  4 

Fixation  of  hybrids,  86 

Fluctuating  variations,  4,  79,  85, 
89,  119,  201 

Four-o'clock,  blending  inheritance 
in,  53,  72;  chlorophyll  inherit- 
ance in,  125 

Frog,  sex  determination  in,  187 

Fruit  fly:  crossing  over  in,  107; 
deficiency  in,  113;  duplication 
in,  113;  gynandromorphs  in, 
187;  _  linkage  in,  97,  105; 
multiple  allelomorphs  in,  112; 
mutations  in,  no;  non- 
disjunction in,  185;  sex-lined 
inheritance  in,  184 

Funaria,  sex  determination  in,  191 

Gartner,  C.  F.,  156 

Gal  ton,  Francis,  12 

Gametophyte,  inheritance  in, 
128,  134;  sex  determination  in, 
190,  196 

Garner,  W.  W.,  132 

Gene,  56,  96,  97 

Gene  changes,  109,  119 

Genotype,  45 

Geothe,  J.,  i 

Goldschmidt,  R.,  200 

Graft-hybrids,  121 


Index 


2oq 


Gravatt,  F.,  133 
Guyer,  M.  F.,  10,  25,  31 
Gynandromorphs,  187 

Hansen,  E.  C,  32 

Haploid,  42 

Hayes,  H.  K.,  57,  61,  64 

Hegner,  R.  W.,  16 

Herrick,  C.  J.,  30 

Hertweg,  R.,  187 

Heterochromosomes,  182 

Heterosis,  162 

Heterozygosis,  162,  170 

Heterozygote,  44 

Homoz}-gote,  44 

Hooded    rats,    Castle's    selection 
experiments  with,  88 

Humulus,  chlorophyll  inheritance 
in,  125 

Hutchinson,  C.  B.,  107 
Hybridization,  evolution  through, 
9,  II 

Hybrid  Wgor,  156 

Ikeno,  S.,  124 

Immunity  to  disease,  inheritance 
of,  20 

Inbreeding,  and  hybrid  vigor,  177; 
and  sterility,  134 

Independent  unit  characters,  38 

Indeterminate  variations,  9 

Inhibitory  factors,  64,  67,  69 

Intersexes,  190,  194,  196,  199 

Isolation,  its  role  in  evolution,  7,  1 1 

Jennings,  H.  S.,  24 

Jimson  weed:   non-disjunction  in, 

114;    tetraploidy  in,  115;    trip- 

loidy  in,  115 

Johannsen,  W.,  44,  84 

Jones,  D.  F.,  157,  166,  107,  i74, 
177 

Jordan,  David  Starr,  7 


Kceble,  F.,  173 
Kempton,  J.  II  .  152,  167 
King,  11.  I).,  iss 
Kohircuter,  J.  G.,  156 

Lamarck.  J.,  2,  10,  11,  12.  10.  28 
LancefKld,  1).  E.,  106 
Lethal  factors,  68,  69,  no,  117 
Light  seed,  effect  of,  13,  14 
Linear  arrangement  of  genes,  07, 

105 
Linkage,  q(\  113,  174,  184 

Locus  changes.  loq,  ii^v  •" 

Long,  W.  H.,  i^^ 

Lotsy,  J.  P.,  9,  61 

Lychnis,  sex  determination  in.  iq3 

Malthus,  T.  R.  s 
Marchal,  I'^l..  and  Km.,  igi 
Maternal  inheritance,  125.  14Q 
Matthiola,  factor  interactions  in. 

70;    non-Mendelian  inheritance 

in,  126 

Mayr,  H.,  31 

]Melandrium,  sex  determination  in, 

193 

Mendel,  Gregor,  37,  56,  157 

Mercurialis,  sex  determination  in, 
196 

Metabolic  rate,  in  sex  determina- 
tion, 188,  195,  200 

^Microorganisms,  inheritance  in,  24 

Mirabilis,  blending  inheritance  in, 
53,  72;  chlorophyll  inheritance 
in,  125 

Moditicalion  of  unit  characters,  S8 

McMlifying  factors,  Oi,  04 

Monohybrid  ratio,  44 

Moore,  C.  W.,  131) 

Morgan.  T.  II.,  97,  nj,  187 

Mucor,  sex  in,  io<> 

Mule,  as  an  example  of  hybrid 
vigor,  lOo 

MulUr.  11.  J  ,  110,  113,  117 


2IO 


Outline  of  Genetics 


Multiple  allelomorphs,  112,  119 
Multiple  modifying  factors,  94 
Mumis,  E.  M.,  32 
Mutation,  6,  109,  119 
Mutationists,  88 
Mutilations,  inheritance  of,  17 

Nemec,  B.,  146 

Neo-Darwinians,  13 

Neo-Lamarckians,  13 

Nicotiana:  self-sterility  in,  138; 
hybrid  \^gor  in,  164;  wide 
crosses  in,  169 

Nilsson-Ehle,  H.,  72,  78,  113 
Non-disjunction,    114,    116,    185, 

199 
Non-Mendelian  inheritance,  1 23 
Nulliplex,  50 

Oenothera,  mutation  in,  6,  117 
Origin  of  species,  Darwin's  theory, 

3 
Orthogenesis,  8,  11 
Oxidase,  59,  64,  67 

Parasitella,  sex  in,  198 
Parasitism  and  sex,  198 
Park,  J.  B.,  138 
Parthenogenesis,  133 
Particulate  inheritance,  123 
Pea,  jNIendel's  experiments  with, 

37,  157 
Pelargonium,   chimaeras   in,    122; 

chlorophyll  inheritance  in,  123 

Pellew,  C.  W.,  173 

Phenotype,  45 

Phycomyces,  sex  in,  197 

Physical  basis  of  heredity,  41 

Physical  correlation,  81 

Physiological   theories  of  sex  de- 
termination, 181,  187 

Pigeons,  sex  determination  in,  188 
Presence  and  absence  hypothesis, 
49,  112 


Protoplasmic  connections,  17 
Puccinia,  inheritance  of  acquired 

characters  in,  t,2) 
Purity  of  gametes,  40 

Qualitative  variations,  6 

Quantitative    characters,    inherit- 
ance of,  72 

Quantitative  variations,  4,  72 

Rabbits,   inheritance  of  acquired 

eye  defects  in,  25,  31 
Randolph,  L.  F.,  125 
Reciprocal  crosses,  38 
Reduction  division,  42 
Regression,  87,  89 
Reversion,  59,  87,  89 
Riddle,  O.,  188 

St.  Hilaire,  G.,  i 

Santos,  J.  K.,  192 

Saunders,  E.  R.,  70 

Sax,  K.,  169 

Schaffner,  J.  H.,  195 

Segregation,  40;  cytoplasmic,  123; 
somatic,  120 

Selection,  84,  88,  112 

Selectionists,  88 

Selective  fertilization,  169,  193 

Sex  chromosomes,  182,  191,  192 

Sex  determination,  181 

Sex-limited  inheritance,  184 

Sex-linked  inheritance,  184 

Shull,  G.  H.,  157,  177,  193 

Simplex,  50 

Smith,  E.  A.,  10 

Solan um,  graft-hybrids  in,  121 

Somatic  mutation  of  genes,  119 

Somatic  segregation,  1 20 

Sphaerocarpus,  sex  determination 
in,  190 

Spirog>'ra,  natural  crosses  in,  129 

Sterility,  131 


Index 


211 


Stocks,  factor  interactions  in,  70; 
non-Mendelian  inheritance  in, 
126 

Strasburger,  E.,  190,  196 

Struggle  for  existence,  its  r6le  in 
evolution,  4 

Sturtevant,  A.  H.,  107 

Superfemales,  200 

Supermales,  200 

Supplementary'  factors,  61,  67,  68 

Survival  of  the  fittest,  4 

Susceptibility  to  disease,  inherit- 
ance of,  20 

Sweet  peas,  reversion  in,  59 

Tanaka,  Y.,  107 

Tetraploidy,  114,  n6 

Toads,  sex  determination  in,  188 

Transeau,  E.  N.,  129 

Trihybrid  ratio,  48 

Triple  crossing  over,  107 

Triploidy,  113,  115,  199 

Unit  characters,  38,  89 

Use  and  disuse,  inheritance  of 
effects  of,  2,  19 


Walter.  H.  K.,  on  inheritance  of 
ac(juired  characters,  14,  17 

Weismann,  August,  9,  13,  15,  17, 

i'"^,  19,  -^5.  77 

W  lieat:  cumulative  factors  in,  72; 
complexmutation  in,  113;  hy- 
brid vigor  in,  169 

Wide  crosses,  169 

Wiedershcim,  W.,  i8 

Williams,  J.  Lloyd,  7,1^ 

Winge,  ().,  123 

Winkler   H  .  i:'! 

A'   chromosome,    184,    191,    192, 
200 

Xenia,  145 

1'  chromosome,  184,  191,  192 

Yampolsky,  C,  196 

Yeast,  inheritance  of  acquired 
characters  in,  1,2 

Zedebaur,  E.,  28 
Zeleny,  C,  no,  1 1 1 
Zig-zag  inheritance,  184 


ntOPERTY  UBUIT 

N.  C.  State  CoIIeflC 


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